For tiny size projects or PoCs, you should place all entities class files under respective folders Application, Data, Models, Domain, etc. (like this)
For small to medium size projects, you should create a new folder for each entity under folders Application, Data, Models, Domain, folders, and place entity specific files within. (i.e. make it modular, using separate folders within same class lib)
For big size enterprise projects, you should create domain/module specific libraries for each then follow the previous step approach for each module. (i.e. make it modular, using separate class libs)
When using MediatR in a bigger project, would you consider this project setup a good practice?