If my tigon and liger-interested friends were right, I believe a liger is almost always bigger than either parent, and I believe he said that a tigon is almost always smaller...
I'm not sure about fertility, but I do know it does happen. Relatively rare it might be, but it does happen.
There are also documented occurences of half-cow-half-moose births, although I don't know how long they live and if they're fertile either.
To be honest, I would expect that the existence of hybrids amongst relatively similar life-forms would point to the ToE being vindicated - if they were entirely different, there should be NO hybrids at all.
If we've all come from common ancestors, sometimes perhaps (through the magic of the genetic code which we're far from fully unravelling) hybrid offspring are possible even when we think it unlikely...
still, it's fascinating.