Don't know about what emoticons ae enabled here at kingsnake.
As for the 10 gallon rule, it is usually meant 10 gallons of WATER, not tank, per inch plus 10 or 20 percent.
Since you need to allow for basking space a tank is often not filled more than 75 - 80 percent.
But even so those are minima. And 9 - 10 inches is NOT the maximum size of a full grown female cooter.
It depends upon the species (and what you call a "cooter) but some get 12 - 16 inches.
Besides, what you may not realize is that turtles get bored very easily. A large turtle that has only a few feet of space in which to swim back and forth eventually becoes lethargic and though I can't prove it, I think it effects their health in the long run. of course this varies form species to species as well, but cooters are active turtles.
So are we talking about Pseudemys?
And if so, which species?
cooters at WCT
river cooters at umich
cooter factoids at nsis
barb reader's crazy collection of cooter websites at Turtle times
If you can't get big tank you could consider a large plastic stock tank or an indoor pond made from a wooden box with a pond liner thrown in.
Many keepers have used one or both of these options for keeping turtles indoors.