The size is less important than the ease to work with and how easy it's to make the holes or hole. If you make an ""air tigh"" closure just take what you already have: a hole saw, a dill bit or a Unibti... You can start with a smaller hole and can make it bigger if not big enough but have the closing flapper made in consideration for a future enlargement if needed. The most important is to take the easiest method for you.
Mine were about 3/4" each, but too low for a good over fire burn, impossble to drill those glasses
but kept the windows very clean as you can see