A 26in wheel with 2in tires is about the same diameter as a 700 series rim with a 700 x 28c series road tire. Bigger than that and you get onto tire/fork arch safety clearance issues so a real 29er uses a larger fork size than a 26in MTB.700 series rims and tires are really only suitable for 26in frames if you restrict your tires to narrow road or cyclocross tires.