A mage in my game would like to have a Roc chick that is about to hatch as his Familiar. The rules on the Familiar edge give guidelines on the size of creature that can be a familiar, and so the young Roc should not be a problem. But, as it grows to a much bigger creature, can it stay as a familiar or would it break free?
There are two ways you can take this.
1) Figure out how fast the roc grows, and if its growth in size categories outpaces his max familiar size, then it breaks free, and he has to find a new familiar. Presumably it would be wild enough to not stick around the party, but if he hasn't been mistreating it then it probably wouldn't actually attack the party either. He would probably know that his hold on it was breaking several days before it actually stopped being his familiar.
2) Being his familiar could magically slow its growth, so that it only bumps up to a new size category when he can have a familiar of that size category. This allows him to keep it as a familiar for as long as he wants (though it will probably never reach adult size).