Largest me and my friends ever printed from a 6MP DSLR was A0.
That was for a Shop's In-store poster.
Since A0 is 8X larger than a A3...it is common to see "grains" or pixels, up close, if u view it within 30cm like reading a book.
But if u display it on a wall and stand 1m distance, viewing it....it looks nice still.
Workflow, dpi, resolutions etc are the same.
Just std Photoshop editing to correct any "flaws", and taken using the max resolution of the camera.
(for A3, even a D2H's 4MP is good enough)