It's basically the largest sedan class available. It will probably fit, but it will be a tight squeeze with that much luggage. The people sitting in the back may have to have a carry-on bag or two on their lap. Depending on how large the suitcases are, there's the possibility that only two of them will be able to fit into the trunk (boot). It should be manageable though.

Note that Avis SFO is a very large airport operation and they will have a range of vehicles available, so they will usually try to accommodate you. You will almost always have the option of upgrading to something larger if you can't find something in the full-size car class that meet your needs.

Hope this helps!