I'm trying to replace a french bottom bracket... the crank is specifying a 110.5mm spindle and I can basically get a 110 or a 113. I basically need to know which to choose.

Thanks a bunch!

.5mm is like 3 hairs thickness, so.............................

so....... 110 then?

110 would be my first choice. If you're dealing with French threading, undoubtably you have friction shifters/derailleurs? In that case, a mm or two shouldn't upset anything.

I'm converting it to a single speed, so no derailers... would the .5mm be a big deal then? I'm just worried about the chain being in line I guess and minimizing chain slip

Either one will do, you're overthinking it.