Within reason, the bigger the better frankly
A larger room will push the modal behaviour down lower in frequency, making it easier to handle through sub placement and EQ. Also, later reflections are more easily filtered out by the brain than earlier ones.
The main reason to keep the size down would be to save on costs. Having said that, a smaller room can also be made to work well with good treatment. You can always make a room acoustically bigger with broadband absorption. Also, if the walls are lossy enough, room modal behaviour is more easily controlled.
There are a couple of good textbooks on this topic, plus lots of articles online. I’m just boarding a flight now but can send you some links later on..