Why make her giant when you can be small? Larger land-dwellers from way back when died out because of atmospheric oxygen becoming lower over time and had to spend most of their time immobile or partially submerged while having absurd caloric requirements. Also, if they took a fall, it would likely be lethal for them, as larger animals like horses never truly make a 100% recovery from a bone fracture (and in many cases, they don't even heal enough to support its weight again). Keeping this in mind, if you fell off her, the absolutely best you could hope for is a shitload of broken bones. You don't want your waifu to become giant and then fall face first on the pavement when gravity slaps her upside the head and then suffocate because something as simply as breathing is now an exhausting task that takes a fuckload of energy, do you?
While the girl being giant is definitely more appealing, shrinking yourself down is way more practical. Smaller living beings are less susceptible to injury from long distance falls. While a 6 ft. tall man has a high chance of dying from falling 36 ft., a man the size of 2 inches falling one foot would be completely fine because of gravity's relation to mass. Height and volume aren't correlated with one another, so if you were around 1/35th your height, your new mass would be much lower than 1/35th.