RE: How do you know when your child is too tall for carseat?
With either seat: The seat is outgrown by height when the shoulders are above the top slot (you may have to feel under the cover to be able to tell), OR when the top of the ears are even with the top of the seat.
The center strap=the crotch strap? I think they are the same length for all of the Britax seats. On the Decathalon you can choose three different crotch strap positions. I never found it a problem on my MA though. The COSCO crotch strap on our Scenera seems much shorter.
Mommy to Gwen 2/7/04 & Nora 4/1/08.