I'm not sure if this is the answer to your question but:

choose RENDER DIMENSIONS from RENDER menu,
select RENDER TO EXACT RESOLUTION
type in the dimention you want
render

Now poser will jump to render window and will start rendering.
After finish your output render will have the dimention you wanted - you can pan it wit left mouse button
or
you can hit the small icon (like face with an arrow) in right top corner of render window and the picture will be transferred to new window.