That isn't exactly how it works. You need the correct bottom bracket type to fit your shell threading and width (which is 68mm english on the Big Dummy),

But then the spindle length is dependent upon obtaining the correct chain line for your bike with your cranks. You sometimes have to take max. chainring clearance into consideration when choosing the cranks and chainrings, however, Big Dummy doesn't have a max chainring spec: you can run whatever size you like.

For a standard bike set up, you can just go with the crank manufacturers spindle recommendations.

Try to match chain line specs too: For instance, if you're using an internal gear hub with a chain line spec of 42 mm, don't use a crankset designed for a 50mm chain line unless you know you can get a shorter spindle to bring the crank and chainring(s) in to the right spot and still clear the frame for instance. Better off to just buy stuff that was meant to play together from the get-go and save yourself trouble.

You can read Surly's talk on the subject here:

http://surlybikes.com/blog/spew/spew...bracket_sizin/