How much should a repo size be?

I think there is a concept of bounded context. It is like a smaller part of your application which can work independently. Often people use this to design micro service architecture. If you are designing a micro service architecture then you should create separate repos for each micro services. Different team can work with different micro services in different repo independently.

If you are creating a monolith application then all your code will end up in a single repo. Often you need some small utilities which are not included in your main application but acts like an extension. For those you should create different repos.

If you are storing everything in a single repo then the size of that repo will be big and unmanageable. On the other hand if you are creating many small repos then also maintaining all of them will be a challenging job. We should maintain a good balance between the two. Bounded context can be a good helping guide on this.