For spring break, we planned an international trip that would require several taxi rides/hired drivers. Bringing our usual forward facing carseat was not feasible. Our daughter is 5years old/41ish inches tall/43lbs. I bought a size small. It was honestly such a breeze to use. We packed it in a backpack and could easily get our daughter in and out of the car. I sat in the backseat with her on all our rides, and she was pretty comfortable the whole 8ish hours we used the vest over the course of a week. I felt very confident that she was safe, and this vest saved us a ton of hassle and stress.I HIGHLY recommend watching several of the RideSafer videos on YouTube to make sure you get the best fit and install. This seat is a bit different from a "normal" carseat (ex the shoulder straps don't need to pass a pinch test, they can be a little loose). Take the time to watch the videos and practice *before* your trip. I did a couple test runs in the house while watching the videos and adjusting straps, then we tested it in the car. At first, I thought it was way too small for her belly, but after some adjustments and watching the videos, it was fine.