Our spa is 7' round. I was concerned it wouldn't be large enough since I've seen so many people with larger spas. But now that we've had the gunite for about 2 months, it seems much larger than we need. I'm glad we didn't go larger. Maybe I'll feel differently once we're actually using it with water, etc.
Primarily, it depends on your use of it and how many people you expect to have. We have a small circle of friends, so for us, it's plenty large. Now, if all these TFP-ers come visit, I might have some serious regrets!!!!! LOL!!!! (just throwing in a different view....). Take care. Suz.