Just personally speaking, I don't like the arch that ends in an abrupt angle, it looks unfinished to me. And if you look at traditional classical arches, you'll see that the diameter of the ach is the same as the width of the opening. In this case the arch would have a radias of 16".
Things to consider in the height: first just the look of the thing in relation to room dimension, ceiling height, other doors, windows etc, and the proportions of opening itself. It could wind up looking too tall and skinny. There's pro'lly some "golden ratio" about arches, but its really just what looks right to you.
Then there's leaving room above for that molding at the celing. neal is gonna put that up, It makes my brain hurt trying to remember to cut the joints upside down and backwards. I can barely miter outside corners much less cope with the inside ones.
Then there's clearance for folks walking thru. If the top of the arch is 80" and you its a full arch half circle, the sides of opening will only be 5' 4" high. Without doin geometry, I think that means you'll have 6' high 8" either side of center.
You might wamt to draw a bunch of arcs on the wall or borrow neal's aluminum strip and see what looks and feels right to you. You can cut a inch wide strip from eighth inch masonite, if you don't bend it too sharp it won't break and should bend evenly. 1/8 masonite can be used to face inside of arch too.
If you have a band saw and spindle sander, it may be easier to build arch as bench work. make your own pre-built instead of buying one. Sandwich 3/4 plywood and cut both at same time. bandsaw will keep edges square, Even best saber saws want to wander off square at bottom cut and more so the thicker the cut. You can make template and cut plywood with router and straight bit. If edges are square you can nail masonite directly to plywood. Lots of glue, nails and clamps.Even better is bending birch if you can find it in your area.Its thin, loosely glued plywood that bends into some relativly sharp curves. By building up a few layers of it, you may not need much blocking directly behind it.
I've made more arches as cabinet maker than as a house builder, but I think that unless the arch is shallow, its gonna be tricky getting dry wall to curve smoothly, flex board is available, if you can find that. Luckily I had a huge bandsaw, 4 foot wheels, watching where i put nails, i could put in solid blocking and cut the entire thing at once then run it past a heavy spindle sander. That don't help you at all but I do like encouraging "tool envy." It makes the adventures I had hauling that cast iron monster 200 miles across country, burning out a trailer bearing just as we reached the city, and watching a wheel croos 4 lanes and the median to go rolling into oncoming traffic worth it.
I probably was no help to you at all, just wasted your time. Mebbe you should ask advice from Robin Hood or william tell or some other famous archer. Don't bother Green Arrow, he's got super powers and his advice for us mere mortals is as useless as mine.
[CENTER][FONT=Comic Sans MS]Measure twice, cut once.[/FONT][/CENTER]
[CENTER][FONT=Comic Sans MS]Look at the nail, not the hammer. Watch the fence, not the blade.[/FONT][/CENTER]
[CENTER][FONT=Comic Sans MS]If you hook your thumb over your belt you won't hit it with the hammer or leave it layin on the saw table.[/FONT][/CENTER]