I actually made such a class just for this called UICountingLabel:
http://github.com/dataxpress/UICountingLabel
It allows you to specify whether you want the counting mode to be linear, ease in, ease out, or ease in/out. Ease in/out starts counting slowly, speeds up, and then finishes slowly – all in whatever amount of time you specify.
It doesn’t currently support setting the actual font size of the label based on the current value, though I may add support for that if it’s a feature that’s in-demand. Most of the labels in my layouts don’t have a lot of room to grow or shrink, so I’m not sure how you want to use it. However, it behaves totally like a normal label, so you can change the font size on your own as well.