I would leave a little extra length in case you add a longer stock, recoil pad, or muzzle break. I put a U.S.-made Bulgarian style plastic stock on one of my SAR-1s and I like it a lot. I think it adds another 1.25" or so to the length.
Make sure you make the case big enough to fit a few mags in it. That way you can keep 3 or 4 in it while its packed away. My FAL barely fits in the case that came with it especially now that it has a scope on it.