Open to any tire size as long as you have SS. I know of a few peeps (like Morrison) doing it on their 26er.
Sorry can't offer good suggestions on good 26 ss race bikes. I would think the lightest you can affort is good. A lot of the good SS racers are using bikes that are either 29'er or 26 but not from manufacturers that normally make 26'er gearies. Does that make sense? In other words, if you looking for a good 26 SS race bike? Look for a manufacter that specializes on making SS bikes instead of a good brand name gearie (like specialized, gary fisher, giant, etc..)
There tons of successful 26er conversions to SS. But most good SS riders I have seen are either on 29er or 26er hardtails with non-traditional brand names.