So I need to know more and a Google search hasn't given me what I need.

If I wanted to print up at 36 x 36 inches, in full colour, how big should the original be for excellent resolution?

My gut would say 36 x 36 at 300dpi, but I see a lot of 150dpi on poster printer sites, etc...

I really need to know as much as I can before I shell out cash on a project.

Thanks in advance!