You'll need a render approx 756x1124 pixels. If you're having it printed, ask your printer what dpi they'd like and render it at the dpi they recommend. At the drop down menu where you can find your render settings, there is also a menu called Render Dimensions. Set everything up there. You can set the width and height as well as the dpi.

There's a calculator here that you can use for future things such as this ;o).

Laurie