If you are writing fantasy, you can do things that won't work in the real world. A person can levitate or fly, change shape and mass, or anything else as long as you stay within the rules you've created for that fantasy world.
However, some real world rules apply. Magic use should take a physical toll on the human wizard since it uses physical and mental energy. You can decide just how much energy is needed in your world, but you'll have to make sure your user has to eat and rest at a certain point.
Many recent novels with vampires, werewolves, and other creatures have presented these creatures as real, not magic. They are genetic mutations, or victims of a virus, or something like that.
If your creatures are real, not magic, then you have to think carefully about their special abilities, their energy requirements, etc., because you can no longer get away with saying that it's magic and doesn't have to make sense.
I'll talk more about that next week.