How do I make a spell with "level trigger"

[chipmush]

ok, I just got this idea:

When you're a hunter, you pet slowly grows each time it lvls, and you cant even notice it until you pass another hunter, who's pet is lowlvled.

That is, when you lvl, you won't notice that you've gotten bigger, but when you see a lowlvled, you'll clearly notice the difference?

How do you make it trigger every time you level?
I was thinking of using the Emberseer Growing spell, but hopefully you can change the amount of growth.
Thx in advance!