Not sure why the pet store owner would say no rats. Rats are generally preferred since it's easier to feed (1 rat instead of 5 or 6 mice at a time). Millions of ball pythons across the world are thriving on rats.
What to feed now is based more on weight than length. If you have a scale, get your snake's weight in grams. If you have no scale, you'll have to eyeball it. You generally want something as big around as the snake is wide at it's widest point.
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