I have a 90 lb Lab in a VarioCage Double Large. It just barely fits in a MYLR but it does fit, you are definitely not limited to the Double Medium. You can extend it a few inches out from the smallest setting, but not all the way. Note you can't just push it straight back due to the side bolts and the curve of the wheel wells, what you have to do is tilt the crate forwards and push it all the way into the cargo area with the back a few inches in the air, then let it go and it drops into place snugly.
My dog can't stand up (he's pretty tall for a Lab), but you don't WANT a dog to stand up while in a moving car, for safety reasons. He can turn around while in the crate, and seems generally comfortable. I have a bed in there and some blankets.
I position the crate with the doors facing forwards, so he gets in and out the back seats. This way, if we're stopped to charge or he's in Dog Mode, he can hang out in the back seat area where he can stand and stretch. The back seats are less of a jump and more cushioned than jumping into the tailgate, as well. Really, I should carry a ramp to avoid hip dysplasia, and I'll probably do this as my dog gets older. For now he jumps around enough on his own that this adding jumping into the car isn't much added stress.
I've traveled tens of thousands of miles cross-country with my dog in the VarioCage, often with 12+ hour days on the road. He loves getting out at numerous different Superchargers in a single day, with all the different things to sniff, and I often find nearby areas for him to run around off-leash while charging. He eagerly hops into the car, crawls into the crate, and when we get home, sometimes he won't get OUT of the car and naps down there for a while! I leave the doors open and he comes up on his own when he realizes we're not going anywhere.
The most important factors, to me, are whether the dog goes into the crate willingly and whether the dog will go back into the crate all day on a road trip. This is much more important than the dog standing up. If your dog has to be coaxed to approach the car, whines, and fidgets in the crate, then you should look into training and perhaps medication (meclizine works wonders for dogs that get nauseous in the car.) Some dogs can only go a few hours or less even with all that. Training tip: I started with just leaving the VarioCage open on my garage floor, and feeding some of my dog's meals and high-value treats in there before I ever put it in the car.
