As someone said, you have your road bike to help you get an idea of what sort of gear inch you want to run - buy cogs that get you close ony our fixie and don't be afraid to play around with the gearing until you get it just right.

Also consider getting a flip flop fixed/fixed rear hub if you want to be able to take your bike on a commute (with a lower gear inch) and on group rides (with a higher gear inch) or whatnot.