Get number of inches -> 8x11, if you want a really high res print multiply by 300, 200 will be fine, or about 100 if you just want to look at it on the screen only. Then multiply and set your image pixel size to those and the res to the number chosen. I.e. 8x11 @200 dpi is 1600x2200 pixel image.

A bit pointless going much above 300 and you will find it limiting a bit much less than 100 or the notional 72dpi that they say the screen res.

The thread also says that you ought to make your map higher than these values and scale it back at the end but that's another story. I.e. go with 600dpi for working on the map until its finished then slightly blur it and resample back to 300dpi. If your not sure about that bit then skip it and do it at 200 or 300 to start with.