You spread your tails apart ready to curse him and feel the magic flow through you. However, it occurs to you that you aren't sure how to perform the actual curse, as a dozen different ideas go through your head on what the curse would be and how to do it the magic starts to make your tails feel all fuzzy, or at least fuzzier. Before you know it, you suddenly feel an explosive release of energy and are somehow thrown onto your back.
As you climb back to your feet, you find to your surprise that you've managed to climb onto two feet, now human again! Well party. A quick look over your body reveals that you are still covered in fur, have nine tails, paws for feet, and a muzzle. However, you can stand on two feet, and you have hands again! So partly human.
Though it's strange, if you're standing up, why is the other fox still bigger than you? And why is he getting even bigger? And why is it suddenly hard to stand on two legs? You fall over onto all fours for a moment, but you're able to stand back up. You realize what the problem is in an instant. Your body is even younger than it used to be. You fall onto all fours yet again as your body grows younger, and you forget how to walk. You're nothing more than a 1 year old baby now.
And yet you keep getting smaller. In a few moments the fox towers over you, and you like a giant. You're probably no bigger than a baby mouse in comparison to him.
It seems in trying to pick a curse and not figuring it out, you've somehow thrown all three curses onto yourself simultaneously.
The big adult fox grins down at the tiny baby nine tailed fox that is you with a glint in his eye.
"Well then, what do we have here."
"Yip!" Was your only reply, for some reason you couldn't quite remember any words.