I don’t really care about the size overall. What I care about is whether my friends are on it or not. That’s why Facebook is great, because you can literally find anyone and everyone (for the most part). Looking for that person you knew back in high school, or a cousin? They are likely on there. Want to find musicians? There are lots of groups in your area that people go to very often to find other musicians. Want to find people to play D&D? Join a d&d meetup group for your area.

Nothing about a federated site precludes any of that from being possible. It may make it somewhat more tricky, since searching between instances can be a bit wonky, but it is very doable. I think the fact that different instances can connect between each other makes things seem bigger than they actually are. You can be on a one person instance, but still be able to connect with everyone else on the entire fediverse. So overall, size is generally irrelevant, other than building a network effect, which I think is very important for any social media site to succeed.