You want to know how large your assemblies should be.
In general, prefer few large assemblies to many smaller ones. If you have a group of types that are always loaded and used together, place them in the same assembly. If you have two or more assemblies that are always loaded and used together, merge them into a single assembly.
The .NET runtime loads a large assembly faster than several smaller ones. A single, larger assembly creates a smaller working set of the application, and Ngen.exe can optimize it more effectively. On the other hand, you may need to split a larger assembly into several smaller ones if the types in it need to be assigned different identities or trust levels.