When I bred mice for a food supply for my snakes, ALL of my racks and 1/4 screen and I never hed a problem at all.
Matter of fact, I have a couple pans set up now ( have a couple snakes that only want live ) that have 1/4 screen.
Another drawback to 1/2 in my experience is, other than hoppers possibly get throught it, is once your food item gets down to the size of the 1/2 opening, it falls through and usually ( not all time, but most ) the mice won't feed on it, they continue feeding from the top........which in the long run ends up wasting food.
1/4 screen would be just fine IMO
Walter
BOUT' CORNS !!