To work out the pixel size, just multiply dpi by image size: so 24 x 72 = 1728 pixels. A 1728 x 1728 render shouldn't be out of the question, although whether your machine can handle it will depend on what you have in the scene. However 72dpi seems low resolution for print; you'd normally go for 150 - 300 I think, though I'm far from an expert.