Go to the upper bar and pull down the file menu. Click on the document setup line to open this window. Click on maximum recommended to find out whats the biggest bryce thinks your computer can do. If you want to do a square render uncheck the constrain aspect and manually change the larger document resolution dimension to match the smaller. Click on 1:1 render resolution to setup your scene. Everything should fit on your 21" screen so you can setup objects. Render at this setting till you are happy with the results. When you get ready to do your poster click on the 1:4 render resolution or whatever the largest magnification you can get. When you render at this setting only the top left corner of the total picture will appear on your screen. The rest of the image will render but won't fit on your monitor. when the picture is 100% complete save the image. You should be able to open the image in a paint program and zoom out to see the whole image. You can click on the 1:1 aspect and change the pixel size of your document resolution directly. If this is larger than the max you will at least start using hard drive as ram and take days to render. At worst you could lock the computer and loose your file. Save a backup bryce file under a different name before rendering.