The size of the hole in the partner will be defined by your size of your mast, and the thickness of any chaffing/padding material you choose to include.  I was able to salvage just the right amount of good firm leather from my leather oar collar kit and chose to use it.  Putting a little suitable wax/oil substance (like used on the oar collars, maybe lanolin based) helped to stop sqeaks.  I do recommend the leather, as it means you shouldn't wear through your varnish, thus preventing any mositure absorption and subsequent discoloration of the wood at that point.  I also place a smal chunk of rubber sheet (about 2x2" by 1/8" thick) under the mast when stepping it, to prevent wear between the mast base and hull.

But the bottom line is that the size needs to be "just right."  The hole should be just big enough to allow mast insertion without binding, yet no bigger than necessary to prevent annoying "knocking" of the mast when the sail is down, or when winds are light and seas are rough.  Of course a hair too big is better than a hair too small.

I think the only way to determine the size is hold-off on cutting the mast hole until the mast itself is finally shaped.  And you need to take a good height measurement from the base of the mast to the height of the partner (lay the partner or a board across between the thwarts) when determining the mast width at that point.