You should match the food items girth with the snakes girth judging by what you've said - it sounds like small rats is about where your snake is at. I would stay away from the gerbils unless you have an unlimited supply and/or your snake refuses to eat rats - balls have a tendancy to fixate on certain food items and if you feed gerbils may get stuck having to feed him gerbils for a long time. One appropriate sized meal ever 7-10 days is a good general schedule. Balls will continue to grow their whole life, their growth just slows down after the first few years - usually by 3-4 years they have reached their adult size and will add very little length but may continue to put on a bit of weight.
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Tosha 
"Nihil facimus sed id bene facimus"

6.34.0 Ball Python (Harry and Fluffy and gang)
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