As Frank showed, if you divide both the length and the width of an area by 2 you will reduce the square footage to 1/4 of the original.
If you want exactly 1/2 of the square footage, you need to divide both the length and width by the
square root of 2, which from memory is approximately 1.414.
For your sanity you could divide by 1.5 instead (or multiply by 2/3). That would make the square footage 44% of the original.
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Unfortunately, this doesn't account for the
height of the project. If you want to cut the entire
volume in half, you need to divide all dimensions by the
cube root of 2, which is very close to 1.25. (Or multiply by 4/5.)
Yucky, huh?
You should certainly be able to "build" the project in software (SketchUp or any CAD program) and then reduce it "with a single keystroke" after-the-fact. But that would result in odd dimensions for every board.