About dpi -   Poser renders to pixel dimensions. The dpi may be embedded in the saved render for applications like photoshop that use it to determine print sizes. You can change it in photoshop to match the printer you use  and it will make the print larger or smaller 

600 pixels is 2" @ 300 dpi 3" @ 200 dpi or 1" @ 600 dpi  
To get a 3" x 5" print @ 300 dpi you should render 900 x 1500 in Poser 
                8 x 10 is 2400 x 3000

pixels/inch  vs dpi is a whole big issue in itself.