Four is the standard, unless we have to forge ahead with a missing player, then it's three.One thing to consider is a smaller group. How many do you normally play with? I’ve found that attention span can be inversely proportional to the size of the group as they have to give other people more air time, can rely on other people carrying them, and have to wait for more action resolution.
I think my idea group size is 3. Any less than that and they tend to get squishy. 4 can be good too.