Take your car to a baby shop and get the to put the various car seats in the back to try them out. It really is worth doing this.

This is what we done. we have a small wagon (pg brain I cann't even remember what 'brand'). and are having a booster (my 3yr DD is bigger then most 4yr olds - 17kg, very tall), a maxi-rider and a s'n's compaq - It is a tight squeeze. If we didn't go with the booster, there were a couple of options available to us - there were generally lesser known brands of car seat, but weren't as bulky.