Weight and height info is displayed by the seat at most stores. Also the info is on the seats. if the car seat you purchase doesn't fit properly in your car, make sure you return it. there's no way to find out ahead of time. I have my child's car seat behind the passengers seat. I believe the middle of the back seat is the proper placement. please look into extended rear facing as well.
Some stores that sell baby furniture, car seats etc, can fit the seat for you. Where I live, you can take the seat into where you register vehicles and they can do a safety check to make sure the seat has been installed properly.