You need three ft around any table to chairs and walking passage.

This would be easier if we saw a picture of the space.

A 63" round would comfortably seat 6 all the time.

If you want more intimate for 4 people, I would go with 47.5" round.

Another compromise is 54" round fixed table. I fit 6 around it comfortably and have a 12' x 12' dining area.

48" round is a good size to seat four on a regular basis, and with the extension you can even fit more than 6 if needed.