Your question isn't stupid.
I believe the "normal" Duffys are 17" tall(at least mine is and he is from Tokyo). Yes the clothing only comes in one size for the 17" bears.
Clothing WDW and DCA have 10" Duffy's that come with clothes. I believe most if not all of them the clothes don't come off. TDS has had a few Duffy's and Shellie Mays wristlet that have had clothes on. I haven't seen one in person. But I think they did them during Halloween 2010. They were cute.