If she kept striking at it but not constricting then its most likely not a feed response but a defensive strike. It probably has nothing to do with the size of the rodent. Ball pythons in the wild do not "decide" not to feed if the rodent looks too big to eat. They simply respond instinctually to a prey item and strike and constrict and attempt to swallow. Also if it was a full-on feeding strike as opposed to defensive there would be no mistaking it. It would be a strike and hold followed but coils and constriction. In this case for a female that's been fasting the size of the rodent could affect her feeding response, but to say that continued striking is definately related to size is premature. Its more likely that she's not yet ready to eat and is threatened and stressed by the rat.