go to Photoshop or anything similiar , type new with 16 x 20 inch and go to image size u can see the pixel size go to poser and set that PS say in image size But if poser can handle the pixel size , try cheating with maximum pixel size poser5 can handle , but with double or more large DPI of ur print project ( 300 dpi in photoshop is enough for print job) , and in Photoshop open new file with 16 x 20 inch 300dpi , open too the file u `ve rendered , select all ,drag to 300 dpi , now u can play with scale to fit with ur project