Retrimming the inside to the new size is the eazy part, it's the outside that can be hard, what's on the outside of the home for siding? If it's vinyl do you have a whole lot of left over pieces. By going with a smaller window every piece on the sides of the window will need to be replaced to make it look right.
And were are you taking these measurement. To get a real rough opening size you need to remove the casing to get an exact measurement.
What type window is in there now? If it's a double or single hung window did you know you can buy Acrilic block replacement windows?
Do you plan or reselling this home or stay in there for a while? The reason I'm asking is window film is a cheap quick fix and there's dozens of differant styles to choose from.