Maybe I shouldn't admit it here on this forum, but, I have other breeds of dogs besides my darlings Navi and Leah.
As all of you well know, Toys throats are sensitive, so, I never leash any of my guys by their collars. Have a harness for each, even my Labrador Kira, and when any of them are transported in one of my vehicles, they are tethered to the seat via their harness.
I just run a Leash (permanently) down between the seat and back, and tie it off with appropriate slack, to allow them to sit or stand and look out the window. That proper slack does not permit them to be thrown forward, in case of a sudden stop. Temperature permitting, I can lower the windows, and I suppose my Suburban looks like a puppy farm, as I drive along. I have eight dogs
And yes, the size difference between my guys, requires different adjustments, and that a given dog has it's regular place in the car.