using the File > Document setup menu (or simply double-clicking the large render button) will bring up the setup menu which allows you to set the size of the current scene. After a render, when you save the scene, Bryce will automatically save the last rendered image as a .bmp. You can use any number of other graphics programs to convert/compress this .bmp into a .jpg. It's been my experience that using File > Save Image As > (anyname).jpg rarely give good result.