Your "grid" in AC3D can be whatever units you want; feet, inches, meters, whatever.
If your result in another application is "way, way too small", you need to figure out what scale it's in now. Does one AC3D grid square = 1/10, 1/100th of a grid square in FPSC?
Once you know that, simply change your
File >Settings... preferences in AC3D such that the "major grid" square is equal to one square in FPSC, and the subdivisions are 1/10 that size.
Here's the default, where 1 major grid = 1 meter.
If this standard set-up is 1/100th of the scale of FPSC, change the first two values to:
Draw grid major = 100.0
Draw Grid minor (snap grid) = 10.0