swobo sanchez bottom bracket
 

So I just bought this swobo sanchez for $175 and that was just the frame. Now I am trying to build up a fixie. I am new to this but rode my friends fixie and thought it was unbelievably smooth. So my question is the swobo website says that it requires a bottom bracket as such: 68mm BC 1.37 bottom bracket.

I understand the 68mm part but how does the BC 1.37 come into play. Would it really matter wether I used a 113 vs. a 115 mm?

thnx in advance :)