She will not outgrow the infant seat until there is less than one inch of hard shell (the top of the seat) above her head. Her legs being over the end of the seat or touching the seat of the car is NO PROBLEM! As long as she is under the 30lb weight limit of the Safeseat and there is more than 1 inch of seat above her head, she is just fine in the seat, no need to move her.
My oldest rear faced until 18 months, when he reached 33lbs. He is now 6yrs old and in a five point harness still, the Britax Regent. It will harness him to 80lbs, he has at least another 3yrs in this seat before he outgrows it! My youngest rear faced to 21 months, whe nhe reached 33lbs. He is in a Britax Marathon, and hes 2 1/2, 34lbs and 36'' tall and still has at least 3yrs before he outgrows this seat, hten he will move to the Regent.
If you are looking for a seat that will rear face your child as long as possible, and forward face beyond 4yrs/40lbs of other standard seats, then steer clear of the 'combo' seats lke the Alpha Omega Elite, Eddie Bauer 3-in-1, etc. They claim to be the only seat you'll ever need, but most kids outgrow the seat in height long before weight, and long before they are mature physically or mentally to use the seat in booster mode. The very top spot on those seats can only be used in booster mode, not in harness mode. So the seats are generally very short, and outgrown in height aroudn age 3 with MOST kids.
You WANT your child to remain rear facing as long as possible, preferrably beyond age 2, and up to 33/35lbs (reaf facing weight limit of convertible carseats). The AAP recommends children now remain rear facing to 2yrs/33lbs.
Remember that any seat is safe as long as you have it installed correctly and use it correctly EVERY TIME YOU ARE IN THE CAR! This means having it installed correctly by a trained professional (CPST) if you can't do it yourself. Having the harness at or below the shoulders rear facing. At or above the shoulders forward facing. Have the harness snug on the child, and the chest buckle should be at armpit level. A carseat is outgrown rear facing when the childs head is less than an inch from the top of hte seat, or the rear facing wieght limit is reached. A forward facing seat is outgrown when the childs shoulders go over the top harness slot, the ears become level with the seat back, or the child goes over the max weight limit.
Here is a list of convertible seats that will rear face your cihld as long as possible, and will forward face your child beyond 4yrs/40lbs. These seats are a good choice because your child will be able to remain in a five point harness well beyond her 5th birthday with these seats, and you WANT your child to remain in a 5pt harness for as long as possible. The bare minimum you want to use a belted booster is 4yrs/40lbs, but its best to remain ina 5pt as long as possible.
Remember that carseats expire 6yrs after the date of manufacture, adn with these seats you will be able to use them for their full life for one child!
1--Britax Marathon/Boulevard/Decathlon. These seats will RF to 33lbs, unless your date of manufacture is 2/7/08 and after for the Boulevard, 2/19/2008 and after for the Marathon and 1/21/08 and after for the Decathlon, then they will all RF to 35lbs. They will FF, IN A HARNESS to 65lbs. These seats run $200-$300. There are small differences between them. The Marathon is the basic model of the 65lb convertibles with a lift lever harness release--with quite a few different covers to choose from! The Boulevard has "wings" called True Side Impact Protection. It also has a harness adjustor knob that will move the harness without having to uninstall the seat. It is about 1/2" shorter than the Marathon and the Decathlon. The Decathlon has extra padding--the infant insert--and a push button harness release and 3 position crotch buckle settings. http://www.britaxusa.com/
2--Evenflo Triumph ADVANCE--harness to 35lbs RFing and 50lbs FFing. It has the infinite harness adjustor (no slots), so you can easily lift two red tabs and move them. This seat runs $129-$159, depending on which model. The higher end model, the premier, has more padding. http://www.toysrus.com/search/index.jsp?kwCatId=&kw=e...
3--Compass TrueFit--- Itl RF to 35lbs (with the headrest on and 22lbs without the headrest on) and FF to 65lbs. From all the other techs and instructors that went to Lifesavers and experienced this seat, they LOVE it!. It seems to be competitive to Britax convertibles. Runs about $179-$189. They have several covers available. http://www.amazon.com/Compass-True-Convertible-Seat-Monet...
4--Fisher Price Safe Voyage Deluxe--Made by Britax, but has been discontinued due to FP not making carseats anymore, so you will likely get an older date of manufacture. Harness to 33lbs RF and 55lbs FF. The only place I have seen it is at this website and runs $129. It just doesn't have all the 'bells and whistles' that the Britax seats have. NICE tall shell! I have one and love this seat! http://www.albeebaby.com/fiprsavodeco1.html
5--Sunshine Kids Radian 65 or 80--These seats will RF to 33lbs (newer ones will RF to 35lbs) and FF to either 65lbs or 80lbs, depending on the seat. These are the narrowest seats on the market, but have the highest incompatability with vehicles due to the low FF belt path. Runs $180-$300. www.skjp.com
6--Recaro Signo or Como--RF to 35lbs and FF to 70lbs. These seats come in a variety of microfiber covers. They have true side impact protection, but the Como is VERY oddly shaped. The shoulder area is 9" of space and the butt area is 11" of space, so unless you have a tall beanpole, this seat may be awkward for some children. I haven't played with the Signo, so I don't know the measurements off the top of my head. They are supposed to go to 59 or 60" tall---I dont' see it. Runs $250. http://www.recaro.com/index.php?id=3312&L=2
Most of these seats have a higher sticker price on them, BUT the seats that have the 40lbs harness limits, that are less expensive, usually only harness the child to maybe age 3-4. AT that age, MOST children are not ready to go into a booster. Bare minimum safety standards say that you can put a 4 year old AND 40lbs child in one, BUT being in a harness is MUCH safer....not just for children, but adults as well.
****Just remember that the right seat for you is one that your child fits in, it fits your vehicle AND you use it correctly each and every time.****