Roller works just fine with 20". I have ravel Trac Technique PRO Alloy Rollers from Performance bikes... (Got it just around $90 after tax and all when they had sales coupon with misprinted pricing somewhere I managed to match up for..)
It is similar to real road effort up to about 17-18mph for my bike configuration (scorcher 20x1.5" on dahon Speed TR). Beyond that, it's easier on roller probably due to lack of air resistance really (but I am not usually going for recover ride or zone 2,3 on the roller anyways).
Didn't really need to adjust anything for it to be useful... I use it when I didn't get a ride out for more than couple of days to keep my leg in tune.