I would say you need at least 12" woofers for that size room - probably 15". Anything smaller that delivers signifcant base will probably be with tuned LF ports and these often sound rather boomy. I have a 25' by 20' room with 9' ceilings and use a sub with a 15" woofer and two left and right channels with 12" bass woofers each.
Good luck!
Good luck!