Oh I see what you mean.
You could scale the images based on the size of the window, but odds are your graphics would start to look bad at some point. You could also just fix the size of the window or give the user a set of pre-selected options that you know you can deal with and let them choose one. Or you could just use a Clear() color that looks ok with the rest of what you are drawing and let space appear around your drawing if the user insists on make the window a lot bigger. Kind of depends on what you ultimately want the user to be able to do. I'm sure other people have some better suggestions.