I put an 11-32 on my 20" Dahon Boardwalk to have better top-end and lower gearing for hills. This necessitated a new/used derailler.

I also have a 16" Dahon Getaway V. I added a 12-28 for more speed and then a 60-tooth front chainring. The chain rubs the frame slightly in the lowest gear, so I think 28 is the limit...but I would like to go to an 11-28 to get just a little more top end, although the 12-28 with the 60 is fast enough for commuting with no significant overall time penalty.