Bottom bracket size: 108mm or 133mm?
 

I'm building up a Motobecane Fantom Cross Uno frameset. If you buy it complete from bikesdirect, it comes with a 113mm bottom bracket. If I'm putting in my own bottom bracket and crankset (SRAM track w/ Truvativ powerspline), will using a 108mm bottom bracket mess with my chainline too much?

What's the rear spacing on the bike? Which SRAM crankset? The one Powerspline crankset I found recommends either a 113mm or 118mm, presumably to fit either 120mm or 130mm rear spacing.

If you can adjust the position of your cog on the rear wheel, you might be able to get the 108 to work but it may also put the chainrings too close to the chainstays.

IIRC, each crankset should have a spec for BB length.

No (as in it will not mess with your chainline)

correct chainline = matched BB + crank, which is what you are doing.

Park Tool Website
If you can get crank to spin with 108 without hitting frame ok. If front derailuer shifts only to middle ring instead of small ring then go to the 113.