You're right, at present the snapshots are a direct same size copy of what's on the screen. I'll do some tests, but I should be able to scale the images to a specified size. I have some questions:
Do you want to retain the aspect ratio of the screen images, or have complete freedom to specify width and height?
The white borders vary somewhat in width and height, depending on the amount of text they hold. The connection speed graph, for example, has a wider left border because it has larger numbers on the axis. Would you want the actual graph area to be defined? (I should say that I'm not keen on doing this, because it would be quite fiddly to get right.)