The DPI setting does not change how many pixels you render. It changes how it would print if you use a printing application that pays attention to the DPI. It is just a number. When set to 72, the default scaling applied to the paper version will be 72 dots per inch.

As far as Poser is concerned the only thing that matters is how many pixels do you want across and down. 1200 x 1200 at 300 DPI or 150 DPI or 100 DPI is still 1200 x 1200 pixels. The only difference would be when printed (with software that doesn't allow you to arbitrarily scale) it would come out 4 inches or 6 inches or 12 inches, for each of those DPI values.

When you explicitly set a dimension, the render will be that dimension, regardless of your preview image size, so of course it may be bigger or smaller than your preview window.

If you choose a different aspect ratio from your preview, then you will not see the same composition in the render as you will in the full preview window. However, the composition you will get is shown in the preview as a highlighted rectangle within the preview window. So by looking at the preview highlight, you can be certain that what you see in the highlighted area (not the ENTIRE preview window) is what will be in the render.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)