bikemig, There just isn't an optimal tire size, in height or width. There, I wrote it.

On my two Cannondale (700C) touring bikes I've used 32, 35, and 37 mm tires. The 35 mm tires were Panaracer Crosstowns and wide enough to almost touch the chain stays whereas my 37 mm Contis have plenty of chain stay clearance. The Crosstowns really bridged paved and non paved surfaces, but were very heavy and an epic battle to mount and remove. The 32 and 37 mm tires feel very similar with a load with the comfort nod to the 37 mm tires unloaded.

My Trek mountain bike has some 26"X1.95" Conti dual tread tires that work great on hardpack and acceptable on pavement. Not so great in mud...

No matter what tire, tire size, or tread is chosen, there are some concessions for riding outside of it's intended element. My next set of tires will likely be 37 mm Gatorskins or something similar.

Brad