At level 10 a ranger's companion grows in size. If I had a wolf at medium size then it goes to large at level 7 would it be ride-able?
Yes.
Yes ... if the ranger is at least one size smaller
This brought to mind my GM's fear with my druid.
I was able to finagle a T-rex into allowing me to take an egg (rolled really well on my diplomacy)
After this bad boy is raised by me, he was afraid I would try to have my "big cat" AC (large sized) ride the T-rex (huge) while I (medium) rode the cat. Inspiring fear (and confusion) into any who saw this.
He would never allow it but it was a thought in his own head that I would ask about it, I'm not that silly though :) However it was a humorous idea