Paula, I went out and bought the Cosco seats for my son when he outgrew his infant seat. My car is rather small (a 4 door Acura Integra) and it fits in there. I also have 2 car seats (DD is 2) in my car and the back seat is really small. You can adjust the angle. A convertible seat will fit differently than an infant carrier seat. My son and daughter were both in infant car seats/carriers and the handle was always pressed right against the seat. The bigger seats are bigger, but are shaped differently. I really love the Cosco seat that I bought because it's the last one I will ever have to buy- it's a rear facing infant seat, a forward facing infant seat and it can be used as a booster seat later on. It was around $150 at Babies R Us, but well worth the investment. You can try them out in your car before you buy if you shop there.
My son is also on the tall side. I think it was Bosunswife who posted that she had to put her daughter forward facing before her first birthday. She is also tall and the doctor thought the risk of injury to her legs if she were sitting rear facing in an accident was greater than the risk of her facing forward before age 1. You may want to speak to your pediatrician about that.
Wouldn't you like to be a Susan, too?