Create the correct size PSD to start an image grid with.

So here's a question that should be easy to answer, I'm creating a PSD with multiple layers, in each layer is a single photograph, the finished product a simple image grid with 4, 6 or maybe 9 squares.

My first PSD was created as a 900px by 900px with the individual layers all set at 300px by 300px, it all worked fine except the final finished product was only about 7 or 8megs.

My next try, a blank PSD set at 1500 by 1500 with 4 images in individual layers, same problem, the final image is way too small for printing, the individual images are each imported into their layers as 91megs files, so the question is, should I create the original blank new document PSD as something like 5000 by 5000 to end up with a large enough file for printing, or am I missing something.

Thanks for any help