What you need to be in concern with is what manufacture can fit your existing rough opening.
If you down size;
You may be in need of a dry wall etc interior filler. Perhaps larger interior casing. Additional cladding needed exterior. For different brands carry different unit dimensions.
If you up size;
You may take away from the integrity of the framing. cutting the header, removal or down size of existing jack studs.
Be in more concern of purchasing a maintenance free window (clad) to meet your existing rough openings.
Happy New Year