The rule of thumb is don't feed a cricket larger than the space between their eyes. I would get small-medium crickets for them. As far as the stress goes, don't handle him anymore, and make sure there is a supply of food for him. have you ever hand fed them? If so, then try that after a couple days of not eating at all. If not, you may have to force feed if he goes 2 weeks without any food. It is stressful for both you and the frog, but it may need to be done. For the next few days just let him be, don't touch or handle at all. only go into the tank for necessities. I know how tempting it may be, but it can be a huge source of stress.