My wife has a britax seat the twists round which is brilliant but very bulky, rear and forward facing once she gets to a certain weight, not tried it forward yet but she drives a Volvo V50 estate so quite a big car.

 

My wife's seat.
http://m.britax.co.uk/product-overvi...-seats/dualfix

My seat
http://m.britax.co.uk/product-overvi...seats/duo-plus

 

You be ok with a new born, just a problem if you want to keep them rear facing when they go into the bigger seat.

 

The boot is plenty big enough to keep the screaming little ******* in
Also stops baby sick on the back seats