I've used a Surly dingle cog, 17/20 paired with a Sugino RD-2 crank with 42 and 39 tooth chainrings on a 103mm BB (recommended width for fixed used on the RD-2). It depends on your dropout length and chainstay length, but I was able to use 3 of the 4 gear combinations with one length of chain. 39/20, 42/17 and 39/17 or 39/20, 42/17 and 42/20.

Make sure you use a 9 speed chain to avoid rubbing on the dingle cog. This of course will mean you'll need a 3/32" chainring.

BB width depends on your cranks, whichever gives you a track chainline is what you should use.