A ioun wyrd is a pile of flying rocks. A mauler ioun wyrd has the strength to carry a rider...but, well, can you saddle one?
The image for them provided in Familiar Folio demonstrates them as having a humanoid shape. This makes them about as difficult to ride as you might expect, and at the very least, you would need an exotic saddle.
Bonus silly question: A ioun wyrd should have at least one ioun stone in it. If it's a mauler, does the ioun stone(s) get bigger too?
I would say that it does, as per equipment increasing in size with enlarge person. Of course, once the transformation ends, so too does the size increase.
Bonus bonus question: A ioun wyrd is a construct. A mauler ioun wyrd can get to size Medium. Constructs of different sizes get bonuses to hit points (in lieu of having a Constitution score). A familiar generally only has hit points equal to 1/2 of their master's maximum. Does a Medium-sized ioun wyrd get the bonus hit points for being bigger?
The hit points of the ioun wyrd itself should increase by its increased size, using the base 5 that ioun wyrds typically have. However, this personal hit point value is still replaced by 'half of their master's hit point maximum', and changing in size won't affect this.
Being a construct, their shape is open to interpretation. You could likely build one as a centaur like mawler and get away with it. You would need an exotic saddle however!
Agreed, the HP won't change due to an artificially imposed limit by the class feature.
However, an Ioun Wyrd that stops being a familiar could up it's HP if it's calculated HP from being bigger would be more, since it's no longer using it's master's HP. (For example, if you buy it as a follower and then get a new familiar).