Many of the small-leafed vining type plants can be trimmed to stay in a small space. Marcgravia
, the vining Peperomia
such as prostrata
; Glassboxtropicals notes the leaf size of plants, so you can look for others that way.
For flowering plants, Sinningia
(at least the 'Freckles' I have) stay small. There are dozens of orchids that are tiny -- I have Pleurothallis grobyi
'Small' that flowers a few times a year and the whole plant is smaller than a ping pong ball. (Oops, you didn't ask for flowering plants. I'll leave this here anyway.