There can be quite a size range in 6 month old ball pythons and there can be quite a size range in what one pet store calls a small mouse and what another one will sell you as a small. Basically just go with what you've been advised about comparing the rodents girth to the snake's girth. On rodents go with the butt/hip area compared to your snake's empty girth and go slightly smaller. If your snake seems quite satisfied with her weekly mouse and isn't hunting within a day or two and she's shedding, growing, pooping and peeing then she's likely just fine. If you feel she needs a bit more prey volume than go up slightly in size but as with anything with these snakes, just don't rush it or make huge changes in anything too fast.