Use this trigger or alter it to how you want. Remember to preset the HeroLevel variable
Events:
Unit - A unit Gains a level
Actions:
Set HeroLevel = (HeroLevel + 1.00)
Animation - Change (Leveling Hero)'s size to ((100.00 + (HeroLevel x 5.00))%, (100.00 + (HeroLevel x 5.00))%, (100.00 + (HeroLevel x 5.00))%) of its original size