Crossed legs aren't an indication of being too long for the seat ... better to have a broken leg than a broken neck, ya know? I believe that what counts is whether or not the top of the ears are past the top of the seat? Well, depending on the seat -- infant carriers require the top of the head be more than 1" away from the top of the seat I think. Should all be in the manual
At least a couple other countries require that all kids under 4 sit rear-facing & there are seats that will hold kids up to 35 pounds rear-facing ... my 2.5 year old is currently rear-facing in a Britax Wizard. She's very tall for her age but in the Wizard she'll be able to remain rear-facing for at least another year or so yet. She bends her knees & crosses her legs as needed, sometimes she flops them over the sides, whatever makes her most comfortable ... she is actually a LOT happier now than she was forward-facing! The Wizard has a recall on it for the straps, but they're providing the right straps no problem ... a cheaper alternative is the Alpha Omega, which does hold up to 35 pounds rear-facing, I believe.
So, anyway, through all this babbling [sorry] I think either of the combination rear/forward-facing seats I mentioned should be fine for ya ... I think they both start at 5 pounds & go up to either 33 or 35 rear-facing & then you turn them around forward facing & will hold up to 65 pounds in the Wizard or 40 with the harness in the Omega & then it becomes a booster, too.
Hope that helps!