My oldest fit into the Britax Marathon until he was almost 7yrs old, 45lbs and 51" tall. My youngest still fits in his at 4.5yrs old, 40ls, and 42" tall. He is on the 2nd to the top harenss slot and has about 2" above his head, meaning he will be able to use th Marathon for at least another 2yrs.
A car seat is outgrown forward facing when one of these happens; the shoulders go over the top harness slot, the ears become level with the seat back, or the child goes over the forward facing weight limit. Until one of these happens, your child has not outgrown the seat and its still safe to use.
You should NOT be using a winter jacket in the car seat. Its not safe. Puffy winter jackets work by trapping warm air between the layers, and in a crash, 3000+lbs of crash force forces that air out causing the jacket to compress to nothing, and that in turn leaves the harness super loose, causing your child to being ejected from hte vehicle or violently thrown around.
If you doubt this, try this. Put your childs jacket on and buckle her in so you think its safe and tight. Then, unbuckle her (without loosening the harness) and remove her jacket, then rebuckle her. all that slack in the harness is how loose it would be in a crash when that jacket compresses. If you doubt the jacket will compress, sit on it. See it flatten form your weight alone? In a crash there are thousands of pounds of crash force put upon it, it will flatten totally from the force.
A few more things - make sure the harness is at or above her shoulders. If its below for forward facing, it will cause spinal compression, very dangerous.So choose the harness slot at or above her shoulders for forward facing.
Make sure the chest clip is level with her armpits, not down by her belly. It is a pre crash retainer and needs to be at her armpit level to do its job in a crash, which is hold the harness on her shoulders.
the harness should be 'snug as a hug', no slack at all. If you can pinch any slack in the harness up by her shoulders, its too loose, tighten it up.
And no bulky clothing or winter jackets in the car seats! My boys are 7 and 4, and we live in Minnesota, and neither have ever worn a jacket in the car seat, and neither have suffered at all from it. They have fleece jackets they wear in the car seat, and they also have fleece blaknets in the car they cover up with if they are cold, or if we are out and don't have a chance to warm the car up. They get in, take their jackets off, get buckled, and then cover up with their blankets.
www.safekids.org to find a CPST in your area to check your car seat installation and use.