Yep, Johnny's right on, a pixel is a pixel, if you want a certain dpi you need the pixels to make it up, and for print, that's usually quite a lot of pixels.
Like he said, you just multiply the physical dimensions of the paper/print, A3 being 11.7x16.5 if i remember correctly, by the resolution you want to print at, a typical print res being 300dpi. You should need to render at about 3510x4950, check my numbers/math to make sure tho.
As for calculating texture size, you'll want to look at the articles section on Leigh van der Byl's site [posted below for your convenience], and read the one called "Size does count!" [Or read them all, because there's some good stuff there.]
http://www.onona3d.com/index.html