My boys were both 20lbs by 4mos old... they were tanks! We moved them to rear facing convertible car seats at that time as well.
If an infant seat fits in your back seat a convertible will too because they sit more upright than an infant seat, providing more room. That being said, some car seats just do not fit into some vehicles. It may be something you have to try before you buy (babies R Us does this)
I can't say enough about how tickled I am to read that you know about Extended rear facing! The AAP, NHTSA, safekids, even the car seat manufacturers recommend children remain rear facing until a bare minimum of 2 and 30lbs, but shoudl remain rear facing until they outgrow the convertible car seat rear facing! A rear facing car seat is outgrown when the child goes over the weight limit or there is less than one inch of shell above the childs head.
Graco Myride 65 will rear face to 40lbs, forward to 65lbs. Its a great seat, excellent side impact protection and very comfy.
Britax Roundabout 50 will rear face to 35lbs, forward to 50lbs. Again a great easy to install seat.
First years TrueFit will rear face to 35lbs forward to 65lbs. Excellent seat, we just bought this one to rear face our 3yr old daughter we are adopting from Ukraine. She has CP so until we fully know her needs she will be rear facing.
If you are having difficulty getting a seat into your car, please find a CPST at www.safekids.org to help you. This is national car seat safety month so you will find plenty of events all over the country right now.