Well you can double up on feeding if they get too small until they are done. If they look tiny next to him, go to the reptile shop and buy a medium and weigh it at home and feed him. See how it looks in him. If it looks like a football, then downsize. If it leaves a small lump, then feed him mediums assuming he takes them weekly or biweekly.
I was feeding my 9 month old retic medium rats as that seemed to be what others here fed but he would go crazy 3 days later and was always hungry. I stepped up to large rats and now they fill him up very well and he is quiet.