Kiddy infinity pro, I preferred the restraint system to a harness. If you google it you can see all the pros of it They make a few other models of seat now, too - this was the only one when DS was going into one.
As an aside though you only need to move them when their heads are right at the end of the car seat, it doesn't matter if their legs hang out. Also, if she's squashed with a coat on, take the coat off - it's not safe to use coats in car seats anyway.
If all this exhausted and she's still too big for the Group 0 seat it might be worth a look at extended rear facing, or at least a convertible seat which takes them to 13kg rear facing and then turns around, because it's much safer to keep them rear facing for as long as possible. Personally I wouldn't want to turn a child FF until they were 12 or 15 months minimum.
How heavy is your DD? Group 0+ seats generally have a weight limit of 13kg and unless she's incredibly tall so the top of her head is level with the top of the seat she should stay in the 0+ seat. It's the safest seat she'll ever be in so don't rush to move her out. Our DD has just moved out of hers at 21 months, although she is small.
When you do move her an extended rear facing seat is the safest option. We have a Britax Maxway, which is expensive but has a weight limit of 25kg (most of the next level seats have a limit of 18kg) so should last a long time. There are lots of threads on the car seat forum talking about extended rear facing.
If you don't want a rear facing seat the next best options are a seat with an impact shield instead of a harness or a Jane Exo seat, which has an internal shock absorber.
I went to the car safety centre. They said to get the britax first class. It's a stage 2 but it can go both forward and rear facing. So my dd1 was able to have more room in the bigger seat but remain rear facing until he reached the right weight.
another vote for the kiddypro one, but I would add with the others that you really don't need to rush on to the next stage at 8 months unless your DC is very unusually large. The kiddypro system doesn't use an internal harness so the shock of any impact is distributed across the whole body, not so concentrated on the neck (with the shoulders restrained by harness, the neck takes the brunt - that's the gist of the theory). When DS and DD first went into them they looked a little small but if you use the supplied booster cushion they are fine. Ours sleep in theirs for hours when we do late night drives up to the North East.
I am not uptodate with that stage at the moment (DS now 2 yars 9 months - and things change in that time), but please keep DC rear-facing as long as possible. DS was rear facing until 2.5 years and I felt a lot safer.
Thanks everyone - DD is probably an inch from her head being level with the top of the seat, she's just average really, 8 months and 19lb 7, not really that long either. Must say I was shocked to be having to think about next car seat as cabriofix says up to 29lbs or a year old!!!
We went for a BeSafe Izi Combi x3 so that dd could stay rear facing til 4, but had a recline option and could be nice and gigh to see out. We've been really happy with it. Dd2 aged 15 months is now in it, and dd1 is in a Triofix (Klippan) as it will last her years.
Both seays from the In Car Safety Centre in MK - brilliant advice, fitting and after sales service.
An inch is quite a long way still - you might be surprised as their growth height/lengthwise slows down quite a lot towards the end of the first year. I bought a First Class for DS when he was about 5 months old worrying he was going to grow out of his infant seat but he actually fitted into it until he was 18 months old! (XP ended up taking the first class for his car when we split so at least it got used!)
If she's under 9kg (20lbs) then it's actually illegal to put her into a Group 1 seat as she won't be the right weight for it and it won't be appropriate for her.
V grateful to read the advice as We are having a similar problem, my 6 month old dd is too big (wide!) for her recarro profi plus car seat and we are struggling to find a bigger rear facing car seat that we are sure will fit our car (it seems that some audis are tricky to get car seats in the back of...). There's no way the current seat could fit a 1 yr old as its advertised to...
million it's worth ringing the mk centre or the essex car safety team for advice, they are brilliant at advising for specific circumstances. Or post in the car seat topic and see what other mnetters with your car have.