Abneycat pretty much covered it: 68mm English, and then match the spindle with your crankset.

So pick a crankset first then match up a 68mm bottom bracket according to the crankset requirements.

On my 2011 Big Dummy I used a Truvativ ISOFlow 3.0 Powerspline Crankset that required 113mm Powerspline bottom bracket. So I got a 68mm x 113mm Truvativ Powerspline bottom bracket. By the way, I actually moved them over from my previous Kona Sutra touring built. 68mm bottom brackets are very common on MTB and touring frames AFAIK and Big Dummy is essentially a mountain bike, and most touring frames use MTB components as well. I don't know much about road bikes and other bikes though.

So if you have a 68mm bottom bracket, just use that bottom bracket and the existing crankset.