I have a set of the 2.5 diameter kreitler rollers. the resistance curve they post is not even close to correct. I was still able to easily spin at 25ish mph doing low z2 back when i actually rode (now im just fat and lazy). Hell, even recovery rides still could average like 18-20 mph. I'd recommend getting the smallest diameter you can, it's literally the only way to train with rollers, otherwise you will spin out in your 53x11 just trying to do tempo workouts.