A3 is double A4 , close to 11 x 17"  at 300 dpi for printing that would be an image size of 3,300 x 5100 pixels

Thats a pretty big image.  You might go with 200 dpi . Just mutiply it out.

You can set Poser to render to a specific size in a new window. Its in render settings .  

You can render directly to file by using make movie, with  a frame length of 1 , and a setting of a series of files. It will render one frame, direct to file.