Things I learned the hard way when purchasing my longarm:
* I can't put it against the wall. I need to be able to check the back, cut threads, or untangle birds nests etc. It happens. You will need more room than you will ever think.
* I didn't purchase new. Longarms are touchy things, and support is a must. Unless you like the frustration of not having lessons, or technical support when things go bad.
* Decent lighting sometimes takes on a world of its own. Lighting....GOOD lighting is a must. And get more than you "think" you will need.
I just went back and reread your post...sorry...you were specifically asking about size.
Get the largest size your "room" will allow....but the other posters are correct.....you won't be able to have a 10'in that space unless it is "open" to another room (i.e. no door....just open space)
Good luck!