I have a large bird of paradise in a 12" pot. I recently found that the plant is just about root bound, and ideally should be moved to a larger pot. However, I don't know if I have space to accommodate a plant that's much larger than it currently is! Would I be able to trim back the roots to keep the plant in its current pot?
Most probably, but the plant might go into a shock, could lose some leaves, or the remaining roots could even rot. On the other hand it could be just alright. It highly depends on the genus/type of the plant as well. A Golden Pothos most probably would not even notice but there are more sensitive plants (from roots perspective) like Peperomia or Hoya that do not really like if you disturb their roots too much.
You could stick with a new 12" pot though, but with a deeper one (if you have space vertically). So there will be some space downwards for the the roots to grow into instead of using a pot with larger diameter.
If you choose to cut back the roots make sure to sanitize your scissor to reduce the chance of any infection, thus rotting.