Ha! When I saw that you were looking at a 56 or 58 and it was 26" or 700c, I knew right away that you were looking at Rivendell. They're the only ones psycho enough to run 26" on that large of a frame. Usually you have to at least hit 54/53 to get to 26" land. God love 'em.

I'll give you a different take: how available do you think the tires you want will be? 26" is just fine unless you want skinny ones.