I use the smallest of the lunch bags for smaller chams. The ones you cant even really fit a sandwich into. As long as you do not have to stuff them in it shouldnt be too small and if it fits in the box it shouldnt be too large. You should be able to use the larger size without any problems but I do think the smaller ones are better because of less room to move around in. I use a single hole puncher and punch 5 to 6 holes in the bags while its still folded together. So 5 or 6 on each side. I also make sure there are quite a few holes in the box. Usually I will punch a pen through each bottom corner of the boxes when using heat packs, cool packs or even nothing at all. If there is a heat pack I make sure to punch a few holes on the sides at the top so the heat can escape. For cold packs I dont worry about the top holes. I feel its easier to kill a cham with a heat pack than a cold pack.