I have a big yellow lab that I like to keep crated when hauling him around in the back of the car, mainly to keep the fur and mess at a minimum. Unfortunately most crates on the market don't fit well into the Four Touring with the recessed rear cargo area because the doors are on the ends but the crate has to fit in sideways. Finally found a folding crate on Amazon that seemed to fit the measurements. The Petsfit Folding Kennel in the 36x23x24 size. I ordered it and can report that it fits perfectly into the rear of the Four Touring. You have to fold the rear seats down. The crate fits in the actual cargo area but you need to fold the rear seatbacks forward a bit in order to slide the crate in far enough to close the lid. So if you don't have rear passengers this is an ideal solution for the Four Touring and I assume it will work for the other prius models as well. What makes it work well is that there is a zipper door on the side of the crate not just the end so when it is fitted into the car sideways the dog and enter. It fits my large 80 lb yellow lab nicely.
My better ½'s car has a special compartment - just for her dog; It's called - the glove box ~ .... but, our 100Lb lab? .... not so much .