I only have 4 squares cut out in the corners of the contact paper I have covering the mesh lid. The squares are about 2 inch x 2 inch. My rooms pretty well ventilated, so I'm sure that gives enough air flow. Plus I have a big hole cut out for a lamp that I was using, but have stopped using it and just cover that with a towel.
You don't need too big of an opening for air circulation really. That's why I wasn't sure why you were buying a rubbermaid container for a cage, then cutting the top open and putting mesh on it. Defeats the purpose of the rubbermaid holding humidity better. Drilling air holes in the sides is perfect for air flow. No need to have any openings in the lid really unless you're using heat lamps.