Define small.
I class the pictus with a captive max size of around 6" as small, but others would class that as big.
Plecs to look into are the Ancistrus or Cheatostoma genera, most of which stay small.
Some Synodontis spp stay small (including the upside down catfish S. nigriventris) though others get much larger and nastier yet can still be sold under the label of upside down catfish.
A number of woodcats do not get past 4-5" and as such are oft considered small. It depends on what you want (a shoal or an individual) and what you are looking for. There is a lot of choice out there.