I thought it would be cool if players would slightly grow in size as they leveled up. It shouldn't be very much, but just enough to sort of estimate someone's level just by looking at them.
I first thought of 0.1% per level, but that would mean you would be twice as big as a level 1 once you reach 1k.
Then, I thought: 0.015% / level. With this, you would be 15% larger than normal at level 1000, or 50% bigger at lvl 3329. (Tomsen)
It'd infuse a sense of power and dominance into higher level players, which I personally think would be a cool aspect.