Well to save you a bit of hunting, here;s the thing...
5 gallon pails have a ridge under. It blocks drainage even WITH holes.
Holes in the sides TOO are helpful for drainage and aeration.
Pharma is laughing because you are going to be losing your marbles... ba da BING hahaha
MArbles are unnecessary. As long as the SOIL stays in there, you're fine. I drill 5/8" holes in the sides sometimes... but i think half inch is about right... no big deal on hole size, better too big than too small though and 1/2" on a big pail like that is TOO small!