C.C.
We had a Britax Marathon and really loved it. Strap adjustments were super easy (with the child sitting in the seat, even), and both kids felt it was comfortable (as evidenced by the fact that they could never stay awake sitting in the car seat!). My older daughter has always been very tall and was able to stay in the seat until she was almost 5. I don't think it was much bigger than any other convertible seat (but I drive an Expedition, so space was never really much of a consideration).