Structuring repositories

I am trying to use repositories in my MVC program designs and I have run up against a problem on how to best structure them.

as an example, say I have an object USers and I have a UserRepository which has functions like getUser(int id), saveUser(Dal.User model) etc...

So If in my controller I have EditUser and I want to display a view that will have a user details input form. so I can do something like this:

User user = _userRepository.getUserDetails(userId);

The benefit being that my controller just deals with processing HTTP requests, and business logic is moved to repositories, making testing etc easier

So, say I want to display a drop down list of possible roles this user could have in my system, ie client, admin, staff etc

is it ok to have a function in the _userRepository called getPossibleUserRoles() or should I have a seperate _roleRepository with a function getRoles() ?

IS it a bad idea to inject a repository for every entity you encounter into your controller? or is it a bad idea to mix entities inside your repositories, making them cluttered.

I realise I have presented a very simplistic scenario, but obviously as systems grow in complexity you are potentially talking of 10s of repositories needing to be instantiated in a controller for every page call. and also possibly instantiating repositories that are not being used in current controller methods simply to have them available to other controller methods.

Any advice on how best to structure a project using repositories appreciated