Regardless of your room size, where do you sit in this space? In front of the system, off center? Do you want to hear, feel bass when you're away from the system. This will really dictate the size of a the sub woofer you'd want to go with.
I have a 8" ported sub paired with floor stand speaker. Slightly smaller room to yours and it does the job for me. Then again, I'm after bass fill. My sub cannot over power the room no matter what I do. Forcing the sub to play louder leads to bass bloat is all.
If there is low or high pass filtering on your SMSL, you can take a pre out to the sub. Else, high level inputs only, which means the sub needs to have this too.
Budget might be the main issue. You can get a Polk Audio PSW10 for that money.
Focus on quality of sub woofer. The size and watts is of lesser importance.