Hi there, it's really really hard to give you a good quality answer, how confident, experienced are you? Before you are asking the question it obviously isn't something that you feel 100% confident with, so maybe this isn't the horse for you.
My own feeling, a well broke horse that has sat for 6 months should not need bitting up to provide stopping power, they may well be rusty, they may need some miles, but I would not expect a bigger bit to be needed. Remember too if she has stood around, then she will be unfit, the fitter she gets, it could be that she is more forward than you want.
“Never attribute to malice that which can be attributed to stupidity”