Wow, this makes token swarms truly horrific.

That was the point.

Ooh, nice idea.

I really don't know about balance, my experience is out of date. I do wonder how it compares to flat boosts (which are also better with many small creatures).

Well, if you have at least three attacking creatures, it's better than the flat +2/+2 granted by most of those. But then, it also costs more; and doesn't grant trample.

It also has a slightly unfortunate interaction with other enchantments that are trying to pump up your creatures. You can't stack too many of them. But this is so good, you probably won't care. And you can still throw instants on top of it.

Whoops. This probably does not do what you think it does. Since this is a 'Whenever' trigger, if four Grizzly Bears attack, this goes on the stack four times. The first trigger resolves and gives bear #1 +4/+4, because four 2/2 bears are attacking. Then the second trigger resolves, and the second bear gets +3/+3... because three 2/2 bears are attacking with a 6/6 bear. Then the third bear gets +2/+2...

For it to pump everything the same, it would need to be "whenever one or more creatures you control with power 2 or less attack, they get ..."

Thank you, Dude. I realized in hindsight I should have included a fix. You are the hero we need.

I still like the idea, but I'm still not sure about the balance. Compare Collective Blessing

I feel like, if you're attacking with four+ small creatures with collective blessing, then you're already doing really well and your problem isn't that you want them all to be 1 bigger. And if you have less than that, then Collective Blessing has a lot of other benefits like being able to block and still boosting your creatures next turn if some of them died. Of course, if you're playing a format with higher life totals, then the additional attack power being the square of the number of the creatures rather than a multiple starts to be a big benefit :)