If this item were to be implemented in future references, I would suggest it Not being a timed purchase and for a single life until Death.
Reason being, some people like the idea of being small/big. If people want to be friendly with it and bounce around and have a blast, then people are open to do that for as much time as they please. People who provoke others and attack others, are bound to be targeted and killed.
I would see this to be a fair shop purchase for a low price (500 Credits or lower) due to it being something users would like to purchase often. New users also would only be able to purchase it 2 times within small amounts of gameplay. So the story and idea can lay out clean.
It's all a matter of Credit purchases. But in my opinion, all this should be given to VIP Members and their use. People who take their time to donate real money (Essentially) In return for our server credits often times buy VIP for a long period of time. Why have an expensive VIP if there isn't too much to offer. It's why I should see it be improved on a few notes. Especially The head resize and all.
In the end, those are just my thoughts on the topic.