25-28c good for cross
I use the big ones for cyclocross since they are almost as large as the challenge tubes. I think they work better than the larger challenge tubes, which can be cumbersome to install. I run 35 psi and think 30-32 is doable. I weigh 160 lbs. I am amazed I haven't pinch flatted considering the abuse I've put these through. But the tubes have nice thick and supple walls and seem to take the hits in stride.