I bought my 12lb Malitpoo, Milo, the SM1 size. It fits him comfortably and he doesn't seem to care when it is on. Actually, he loves it when I put it on him because he knows he's going for a car ride! The leash is just long enough so that he can sit, lay down, and even stand up to look out the window while he is in the back seat. I just use a caribiner to attach the leash to the seat belt or child seat hook depending on which car I use. With the harness, Milo also seems less nervous (less huffing and puffing) and I feel more secure that he would not fly around during a hard stop or accident. The over quality of the harness is also excellent, not the flimsy kind I see at the local pet store.