I'm not sure what you mean about "safer." If you really want to have a document who's resolution matches the nano-preview window, you could do a screenshot of the main interface, then crop down the image in an image editor to see the height and width of the nano-preview. Once you know these, you can find the ratio of heighth to width. Enter that into the Aspect Ratio in the Document Setup in Bryce. The document setup has a bunch of different presets which will produce different looks in the document resolution. I don't know offhand if any match the nano-window aspect ratio. You may just have to enter the ratio youself and play with the document size/resolution.