Try this:
Create instance variables : PlayerWidth & PlayerHeight and set them to original size
Then in event's
-Is Player moving, every tick :
---set player size: Width to: varPlayerWidth + (Y/n), Height to: varPlayerHeight + (Y/n),
where "n" is a number you will divide Y with so you won't get extremely huge character.
You could try using scaling too, which will probably work better, but can't remember how to do it from top of my head.