What’s the max project radius? What if I need it to be more?

The maximum radius of a project is 5 km. It is defined by the first site ever uploaded or created in a project. In case you need this to be bigger for a certain project, we can study your case to see if an exception can be made. 
To be able to use new sites in the same project just make sure that their center point (not the actual elements) is within that initial radius.