You are a Metamorphmagus, meaning you can change your body and appearance at will. You start out only able to do superficial changes, but with a bit of work you can change your looks however you like, and once you’ve mastered that you steadily become able to control each and every aspect of your body, internally and externally. You must retain a humanoid shape, but you can be any height between four and eight feet as an adult, and of proportionate width. Beyond that, go nuts. Colours, shapes, sizes, gender, age, everything is wide open, and hardly any limits apply.
Holding your transformations requires little to no effort, to the point that you can even sleep or fall unconscious without losing it. And unless you go really crazy, your magic will prevent any of your more ‘unconventional’ transformations from killing or seriously harming you, and will warn you if something you’re going to do is beyond its ability to mitigate.