50px per 5 foot works fine in maptool, but then that's for practical issues which include sending it remotely - which you don't need to worry about. If you just want it for taking screenshots, then figure out the smallest area you'd want a screenshot of - say a 40' wide area - then make sure your image looks good at that resolution on your screen. I'd guess that for an ENworld screenshot you don't want to go to anything wider than 1000px, so that would give you a base resolution of 1000/40=25px per 5 feet. If you wantt o take a screenshot where the map shows something 20' wide then make sure your map looks good at 50px/5'. Any detail above this is only useful if you wnat to zoom in on a single square, or small group of squares. Nice for players within a VTT, but not really an issue if you're the one deciding what gets captured in a screenshot.

On a side note - maptool has 'export as player' and 'export as GM' functions, as well as the ability to place grid reference numbers along both sides of the map. Handy for pbp games, ig this is the use you are putting it to.