Last year a friend of mine's nephews had left some little cups about the same size as the water dish for my whites (about 2x3) outside and a few days latter they found frog eggs in it.
I had my green and gray in a 12x12x12 and they seemed to do ok but before her passing Helly was getting a little overweight which tells me it wasn't the best place for them. Many gray's keepers around here keep several in a 12x12x18 and as long as there are plenty of places for them to climb and hide I'd say that 2-3 could share that size.
Oh no matt!!! What did you have him in? Was the lid not secure or something? I hope you dont have a cat... >> Put a dish or two of water out cuse he'll be looking for water if he's been out all day. Turn the lights out cuse he wont come out while there on (he'll think its still day and keep sleeping). And you can use a flashlight to find his eyes in the dark cuse there eyes with shine bright when hit with the light. It'll also temporarily stun him which will let you catch him easier. Look in damp, dark, and warm places.
I hope you find him! Poor little thing.