Bottom bracket length without knowing front chainline (fixed gear)
Presuming a replacement is necessary, and not a new build, can you remove the cranks and measure the the spindle length?  You can do it with the BB still installed if the caliper arms are 'deep' enough.  

While you are in there with calipers, its probably worth getting the BB Shell diameter and width too.

Another variation is the thread's hand orientation (left/ right).  I forget now side is the variable one.