I recommend both. The 12" is portable. It'll fit in most gym bags. The 36" allows you to just flip over to work the other side.
The axis foam rollers are very durable. One coach I know says he's had his for 5+ years and it's still in the same shape. Mine have held up very well, much better than the typical white biofoam.
Also, watch this to learn about how to make your ankle more flexible:
It's gonna hurt like hell for a couple months. You probably won't be able to put your full weight on it. That means you definitely need it. Begin an aggressive regimen of SMR and static stretching. At least 3x/week.
__________________
I like the baby Jesus. The eight pound six ounce baby Jesus that didn't even know a word yet, but was all cuddly and omnipotent. -- Mike Huckabee