I regularly do 8 x 10 @ 300 ppi so thats  2400 x 3000 in Poser.

I did a 36" X 24" Poster but that was 120 ppi.

 

If I did the arithmetic right 7200 x 7200 at 24 bit color and 300 ppi works out to 155 megabytes of memory to store the image. If your system is low on physical memory the application will crash.

Waht system are you running on ?