I usually use the rule of, you have to have seen the thing you want to shapeshift into and probably interacted with it.
For my players, I have one that can shapeshift, and we went through every creature he has encountered so far, and has a list (if you look at the NPC/Creature post on here, that is where that evolved from).
You could also treat it almost like a Blue Mage in that sense, you have encountered (or fought) it. Maybe a Logic/Learning roll to understand it so you can turn into it.
Otherwise, use the roll for Companion Tier 2/3, and don't make it any more powerful than that for a creature. This is dependent ton the power level of the Roll to shapeshift as well. Higher levels can possibly access higher creatures.
Really up to the GM to decide the limit. Afterall, you could transform into a dragon or T-Rex at some point, right?