Sue-Ann, I really think a 48" diameter round would work for your space, given that you don't plan to have any other furniture in the room. A 48" round will seat 6 people as long as it is a center pedestal table. It will leave you 30" from the table edge to the wall (on your shorter wall), which is enough room, usually, for a person to be seated at the table and another person to pass behind them. A 48" round is a great size for 4 people for every day use. I'm attaching a scale diagram that shows 6 chairs at this size table, as if people were seated. When you don't have all 6 chairs in use, you'll want to pull two chairs to the corners of the room and push four under the table. Does that make sense? I do agree with the suggestions of tape on the floor. In fact, we usually recommend to our clients that they cut a piece of cardboard in the shape of the tabletop and rest it on an end or side table in the room. This will give you a feel for how the table feels in the space in a more 3-dimensional way. If you want a little more space in the room, and don't need to seat 6, go to 42" and you'll be able to seat 5. I hope this helps.
Q