I routinely render at 5120x1600 pixels, for use as desktop backgrounds on dual 2560x1600 pixel monitors.  I don't know of any arbitrary pixel limit in Poser.  I suspect that what your problem is not a limitation of Poser but rather that you are reaching the RAM limit of your machine.  The total render burden could roughly be described as RAM needed = {scene complexity}x{material sophistication}x{render quality settings}x{pixel dimensions}.

Poser 8, being limited to a 32bit RAM allocation -even on a 64bit machine- would hit the wall long before Poser Pro 2010, which can exploit as much RAM as your machine holds.

Big Poser Pro 2010 render used as desktop background:

Cspear's  suggestion should work - I've had stuff printed at much less than 300dpi and it looked fine. 

Chikako, if your pz3 has only content which you can redistribute (or which I already have), I'd be willing to have Cameron take a crack at the render.  She has dual hex processors and 96Gb of RAM.

Poser 12, in feet.  

OSes:  Win7Prox64, Win7Ultx64

Silo Pro 2.5.6 64bit, Vue Infinite 2014.7, Genetica 4.0 Studio, UV Mapper Pro, UV Layout Pro, PhotoImpact X3, GIF Animator 5