I've used just about everything (except greenboard and ordinary drywall) on bathroom walls around tubs.
And, with utmost respect for Launchpad, please don't use greenboard on a shower wall. It's supposed to be water resistant, but on a shower wall you need a water-PROOF material to set your tiles over, not just a water resistant material. You WANT to use a cement board like Hardibacker in wet areas like showers, or a gypsum product specifically recommended for wet areas like Georgia Pacific's "DenShield".
Is cement board unusual for bathroom walls?
Not if the tile setter is doing quality work.
The reason why tile backer panels typically come in 5 foot lengths is because that's the standard length of a bathtub. Standard bathtubs are 5 feet long, but the tile backer at the front and back (and side) of the tub should stop about 1/8 inch above the lip of the tub and the ceramic tiles overhang that lip. Because of the thickness of the tile backer at the front and back of the tub, you typically need the tile backer panel at the side of the tub to be a little narrower than 5 feet. But, it's good that they make it 5 feet wide because walls aren't always straight, and you can always cut it shorter, but you can't stretch it to be longer.
Your bathtub will typically only be 32 inches wide, and you really don't need a tile backer on the walls beyond that width. (But, I would definitely put tile backer on the walls BESIDE the bathtub because water could be splashing on the wall there. That is, on the bottom 16 inches or so of wall BESIDE the tub cuz that's a potentially wet area too.)
Hardibacker is probably about the strongest tile backer you can get, but you should be aware that it dulls metal cutting tools VERY quickly (faster than other cement boards). What I found to be about the best (although a labour intensive) way of putting it up (and this method works well for any cement board, not just Hardibacker), was to predrill 1/8 inch holes in the tile backer where the studs would be with a 1/8" masonary bit, and then countersink those holes with a grinding point of the kind seen second from the left in this photo:
Mount that grinding point in a percussion type hammer drill and have the drill in hammer-drill mode for best results. You'll get a nice conical countersink at each hole in your cement board.
Then, when all your holes are drilled and countersunk, put up your panel, predrill through the holes into the wood stud with a 1/8 inch X-tra long drill bit, fill each countersink with thin set and then drive a #10 stainless steel flat head screw (or an ordinary drywall screw) in so that it's flush with or under the surface of the Hardybacker. The thin set in the counter sink will accomodate any difference in shape between the screw head and the countersink, and the rest of the thin set will come oozing out of the hole and can be scraped off with a putty knife.
I think I did it that way because I used ordinary drywall screws for some of my bathrooms, and the heads of those screws would come popping off if the cement board was too hard. (Wonderboard was notorious for that years ago; some areas would be hard as granite, and some were so soft you could crush it in your hands.) So, if you're having any trouble using any other fasteners, you can always use this method.
If you consider countersinking your holes as described above, you should be aware that the grinding point you use will wear down on any cement board you're trying to countersink, but you can restore the grinding point to it's original shape turning it in an electric drill against the SIDE of a spinning grinding wheel as suggested in the second of these two drawings:
The blue thing would be the grinding wheel, and the grinding point would be conical instead of spherical and would turn in the opposite direction as the grinding wheel.
Anyhow, you can probably buy Hardibacker in 4X8 sheets if you phone up the places listed under Plaster & Drywall Wholesalers (who WILL sell to you because there's no such thing as a drywall retailer who's gonna bark at him for stealing business because the hardware stores buy directly from the manufacturer just like the wholesalers do). But you really don't need it in that size. Just use one piece vertically at the front and back of the tub enclosure and two pieces horizontally at the side of the tub enclosure, and maybe Georgia Pacific DenShield above that to the ceiling.
Denshield is MORE than water resistant enough to work well in a tub shower enclosure, and I wouldn't be reluctant to use it instead of Hardibacker. However, truth be told, Hardibacker is stronger and would stand up to the occasional impact a lot better than Denshield. But, I'd use Denshield from the top of your Hardibacker to the ceiling without concern. I wouldn't use greenboard or drywall anywhere's around your tub cuz it'll get soft and weak and start to bend if it does ever gets wet, and as soon as it bends enough for a grout joint to crack, that's the beginning of the end of your tiling.
Also, I wouldn't try to butt the side of your Hardibacker up against drywall in the middle of a stud. You'll probably find that the studs in your bathroom walls aren't straight enough to do that with good results. I'd use 3 inch drywall screws to fasten a piece of spruce 2X2 lumber to the SIDE of a stud to provide a luxurious amount of room to fasten the Hardibacker to the spruce 2X2 and drywall (or whatever) to the stud beside it. That way, if your studs are as crooked as some of mine were, you can still get decent results.
You can also expect that some of your wall studs will be twisted, split and warped, and then you can just clamp some boards across the face of some studs at the top and bottom of your wall and install the 2X2's snug against the backs of those boards. That way, when you put your Hardibacker or drywall up, they should meet flush at the joints.
Also, you don't tile over the Hardiebacker. Ceramic tiling is not as water proof as you might think cuz the grout is porous. You put up your Hardiebacker, tape your joints and fill with thin set mortar (or drywall joint compound, depending on where the joint is), roll on your waterproof membrane (like RedGard) and tile over that. That way, if moisture ever does get through the tiling, it won't get to the wall studs.
Not sure if this helps you figure out what you're missing. You can buy Denshield in 4X8 sheets, but a 3X5 sheet is more practical for tiling around bath tubs, so that's why it's more common.
Also, I don't know what your game plan is, but I've tiled the walls in well over 20 bathrooms, and what you should consider is drawing a horizontal line above the tub all the way around the enclosure with a spirit level. That height of that line above the tub should be about 1/2 inch or so less than the height of the tiles you intend to use. Fasten STRAIGHT wooden moldings to the wall so the top edge of the molding is level with that line.
Now, do all your tiling above those wood moldings by setting your tiles so they rest on those wood moldings. Remove the wood moldings and cut the bottom row to fit down to the tub LAST. That way, even if your tub isn't installed level, your tiling will still be horizontal and vertical. (It's better to have the tiling horizontal than being equally out of whack as the tub. That's because no one will ever notice that there's a 5 inch tall tile in the bottom row at the front of the tub and a 5 1/2 inch tall tile in that same row at the back of the tub. Human beings are simply not that observant, and in both cases the tiles look to be about the same size. However, if your tiling isn't horizontal, then your vertical grout joints are going to lean toward or away from the corners and you might have to cut "sliver" tiles to fit to a corner or have an inch wide grout joint near the ceiling if you set your first tile where wall meets tub.
1/2 inch is about the most I've seen any of my own tubs be out from front to back, but check all 4 corners just to make sure because they haven't invented a tile stretcher yet.
You can see some pictures of my bathroom wall tiling on my web site at: