What you really want to look at is something like a Surly Long Haul Trucker (which given frame size will take 26" or 700cc) or the CrossCheck which will go upwards of 700 x 45. You've got to build em up yourself but to me that's the fun of it.

Or you could find a decent MTB frame and convert it like I did for the FRT (Fat Tired Rider). I've got a Surly 1x1 frame that's destined for fixed gear-dom.