I'd repost this in the Nurseries and Baby Gear forum. There are some car seat experts on there that can give you good advice and maybe even specific links to your car model and what fits best.
My advice, though (since my husband and I are both tall) is definitely go to Babies R Us or someplace that you can do test fits. Sometimes what seems like it should fit and what shouldn't does. If I knew then what I know now, I wouldn't have even bought the car we have now!
Also, you didn't mention, are you tall as well? If so, and you expect to have tall children, it generally isn't recommended that you choose Britax. Not because they don't make a good, safe product, but the shells are shorter so your kids will grow out of them quickly. I had a Britax Marathon that I had to get rid of because my 10 month son was already really scrunched-- it was just a bad fit. We got a Clek Foonf, which I love and fits him great but I wouldn't recommend to everyone because they're pretty expensive (and you need 4!) and rear facing, you'll never get two of those in the back of a Malibu and still want to sit in the front. It's a big seat and sits up very tall RF.
One recommendation that is normally given for people with smaller cars (although your car isn't small, but you need the extra room) is the Combi Coccoro. It's one of the most compact rear facing seats available but, again, if your children are tall, they may grow out of it fast since it's a smaller seat.