You want to make the holes so that:
1. They allow air to circulate in the enclosure
2. They prevent lots of heat from leaving the enclosure
3. They keep the humidity that you want

If I were building a rubbermaid enclosure, I would set the enclosure up completely and record the temps and humidity. To lower the humidity and temps I would begin placing holes in the enclosure until the needed levels are obtained. As far as a bp breathing, they really do not consume alot of air so the holes that you put in to adjust humidity and temperature will usually work well if they allow air circulation.