All depends on what you are planning to put inside your bag.
For me , bigger is better, to a degree.
I put in 3 racquets with covers so they do not get banged around, tennis clothes, towel, soap, deodorant, 2 water bottles, snacks, phone, hat, sunglasses, tennis shoes, sun blocker, a few tubes of balls, tape measurer (net typically not set properly), room for another racquet that someone drops off to be strung, etc. etc. so it all adds up, and if you go too small, say goodbye to the zippers when you try and zip it shut.