I use 1/4" for tops, 1/2" for feed hoppers.
A while back, I kept finding asf weans in the sticky traps I have out in the floor in the snake room. I couldn't for the life of me figure out where they were coming from. Then one night I witnessed one sitting in the top of the feed hopper of its cage, on top of the lab block. They were squeezing through that 1/2" wire once the food got low enough and taking off. Problem has since been solved by placing stoneware bowls I no longer use over those openings.
The little ones can definitely squeeze through 1/2".
