I use the 2 in size about 2 1/2 in deep. Found that works for me but don't paint many larger than a 1/4. Like PePop I built my own. For the cost of one manufactured unit you can build a whole rack of cups. Do yourself a big favor buy the cups from TJ with filter or the disk and glue on like PePop said. Best thing I did for painting. Several videos on how to build on YouTube and somewhere in here Fatman has a real good thread on building fluid beds.
Proud Member of Team Geezer
Charlie Weaver USN/ENC 1965-1979