First things first....are you going to have a rug? A rug should be 2' bigger all round the table for it to fit properly. So let's say you buy a 9' long table, maybe 42" wide. You would want a rug 11' long so that table probably would be too big.

A square table may be the way to go. 6' x 6' on a 8 x 8 rug would give you lots of room, seat lots of people and still let a lot of floor show around the rug. (accounting for the 2'6" piece in the corner).

For this small space you might consider a round table which can seat more people. Will this room have a buffet or other pieces that take up space because this also will help to determine the size of the table .

Remember, you need room to move around.

It's important to remember also that you need to be able to move around the table when people are sitting at the table. The chairs will be away from the table more when someone is sitting there and you don't want to have to squeeze behind someone to get things. A round table is a good idea. Some of them even have an insert that expand it into an oval. Good luck!