It is two ways to do that: 1. is to have 10 different units for each level(this is the hard way)
2. is to make a trigger that simply makes the hero animations bigger
Event (the hero you want growing) Gains a Level
Conditions
Actions Animation - Change (triggering unit)'s size to ((100.00 + ((25.00 x (Real((Hero Level of (Triggering unit))))) - 25.00))%, (100.00 + ((25.00 x (Real((Hero Level of (Triggering unit))))) - 25.00))%, (100.00 + ((25.00 x (Real((Hero Level of (Triggering unit))))) - 25.00))%) of its original size.
this will make the hero 25% bigger for each level. if you want the hero to be even less bigger/or bigger just change all the 25.00 to the wanted number.
i know this will work and if you need more trigger help just pm me.