Honestly, this kind of thing should be done automatically by whatever app they sign up with. It's not something that the average person will really know how to choose.
Because it's not only number of users, it's also number of connections to other servers, how many federate with it. Most people don't wanna use a social network where they have to type exactly what they want in to search it every time. Never mind the different policies about posting and de-federation that can completely change how the network works for each user.
It's the disadvantage of decentralizion, and we've gotta band-aid it somehow.