If we're going with plate, it is usually custom-made to whoever was gonna wear it. There's no such thing as a plate armor that'd fit anybody and everybody. So in this case, it won't cost any more to create hers than a regular-sized person. If we're going with less constricting options, those are far easier to modify. And are usually done anyway upon purchase. Kinda like you could have your pants length adjusted before buying. It's artificial detail, but it actually subtracts from the world. Handwaved options are usually preferable since it's not susceptible on these types of nitpicking (which, it is, I admit). It just opens the gates, y'know?