When you render to disk also remember to change the render name. If the name of your original file is "spacebattle" add something to the default name like "RDspacebattle" for render to disk, or "spacebattlebig". If you render to disk and make a big bitmap with exactly the same name you will overwrite the default preview image for your bryce scene. The next time you open the file bryce will try to load this large file. If you are already pushing your memory you could find that you cannot open bryce again without crashing. All is not lost, even if this happens you can just rename the bitmap after the fact. Bryce won't be able to find it and will make a new preview image.

Read the making of siege by flak. http://www.3d-digital-wasteland.com/tut_siege.html  You can render ships and masks one at a time in Bryce and apply them to 2D plains without buying photoshop or other image tool. This will allow you to get many more ships into your render.