Quote - rty - Sorry, but what about the combination Constrain Aspect Ratio and Fit to Preview Window doesn't work for you?

The fact that it's pulling the piano towards the stool... In a normal work flow, you would size the preview (your empty canvas), and at the end tell Poser to render it in that same size (or double, or whatever) - just like you did till now in Poser.

Now i have to undock the preview, frame my scene as I would like it to look like, get the pixel values that gives, go enter them into the render menu, go back, dock the preview, and at last work... And if I decide afterwards I want it to be a little wider or narrower or higher, here we go again...

It's not that it is impossible, it just feels clumsy. What lacks is that the preview size can be changed inside the docked preview pane.