You can get Huffyuv [here](http://math.berkeley.edu/~benrg/huffyuv.html).

It might be a little tricky to get working, especially if you have WinXP, but I haven't had any problems with it. Take note that Huffyuv defaults to a YUV colourspace, whereas Poser renders to RGB, so you must configure Huffyuv to use RGB, as shown above. You must also enable alpha-channel support in the codec since it defaults to alpha-channel-off.

I haven't heard about the maximum render size. The largest I've ever rendered in P4 has been 1600x1600, so it hasn't been much of an issue for me yet.