Changing cricket size for Leopard Gecko?

[Kevin]

I have had my leopard gecko since mid October and I got him as a baby so I would guess he is about 2.5 months old. I feed him everyday and over the last month he eats like a dozen small size crickets a day. This is getting to be a problem because Im running out every few days and they arent properly gut loaded. At what point should I switch him to large size crickets or should I just buy from petco online and get a diff size? They dont just sell two sizes they come in pinhead ,1/8"1/4,3/8, 1/2. 2/3, 3/4" and then full size so what would be the best bet? If i had to gues sid say his head is bout halfinch wide? I just want to fill him up easier