Custom posting system

I just installed WP 2.0.2 but I want to customize it a little. Here is what I want:

Every registered user can write a post which will not be published and will be hold for approval. So far so good. The problem is that I don’t the user to see just the “write”, and the “profile” pages, without the “dashboard” and “manage” pages. Also, I want the user to be able to upload a file of certain max lenght.
Is there a plugin that will help me to obtain this? If not, what files do I have to edit?
Also, where can I set the maximum file size for upload?