To reduce the requirements of the program on RAM make sure your wireframe resolutions are only as high as you need them (this is for display purposes only since I don't think that affects file size. For file size make sure that the terrains you are using are only as high in resolution as you need, a 1024 terrain takes up a lot of space. Textures will also increase the file size depending on what resolution they are. As far as what the max is, I haven't reached that high yet. My seadragon pic I'm working on is only around 50 MB with a few million polygons and hundreds of objects.