In fact, everything is simple and directly depends on actually printing device. More particularly, the number of ink drops which causes it inch paper. For it is in the same parameter DPI (Dots Per Inch) .
Suppose the resolution of the printer on which to print the render - at 300 DPI. You say that the print format of the A4 , that is 210 × 297 mm. If A4 round in inches , is 8,27 × 11,69 inches. Total: 8.27 × 11.69 * 300 * 300 = 2481 × 3507 px. 🙂