what size mouse/rat should I be feeding?

I've had my snake for about 3 weeks now and he's been eating great, didn't have the greatest shed but I helped him and got it all off of him using the soaking method on these forms actually. besides that, I'm not sure I'm feeding him the right size prey amount. the store I got him said he was 5 or 6 months old but I feel like he's pretty small for that. I only have a scale for humans so unfortunately I haven't been able to weight him yet. I've been feeding him regular sized adult mice every 7 days, here's a few pics of him and the widest part of his body.
http://uploads.tapatalk-cdn.com/2016...42ffc774d9.jpg
http://uploads.tapatalk-cdn.com/2016...f4ebd8c81e.jpg
http://uploads.tapatalk-cdn.com/2016...c9c48eb597.jpg


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Rule of thumb is 10% the snakes weight or as thick as the thickest part of the snake. Looking at the pictures and trying to compare the size, I would guess something along the lines of a weaned rat or a small rat. Go to whoever you get your rats from and eyeball about how the thick the thickest part of the rat is which will be the rear hips. Also remember, the fur is going to make the rat look bigger too. And I would try to switch him to rats as it will be cheaper and easier when he gets big to feed him 1 appropriate sized rat vs something like 4 mice.

He is big enough for small or weaned rats.