Looking for information on how to make a familiar increase its size, in this case, we're dealing with a bird familiar. Mauler seems like the easy fix to make the hawk familiar medium sized, but is there a way to make a hawk/bird familiar Large enough for a medium creature to ride?
If this isn't the best way of going about this, what about animal companions?
Secondary question, is there an animal companion version of "Merge with Familiar"?
Well, there is a feat specifically for you to ride creatures that are the same size as you. Undersized mount.
Animal Growth spell with Mauler could make it Large. Not permanent but anything that would be wont be cheap.
Enlarge Person could work as well, but not as powerful, with Share Spells feature.