I think I've got it:
There you go! Direct comparison of the Window Frame array to a text string array.
This particular macro centres the front window on my 24" monitor, and makes it one of two sizes, based on what size it was before. The thing is this: Many Web sites are now wider than is comfortable for extended reading. If that's the case, I can easily make it my preferred size, and then back to the wider window for 'normal' browsing.
The plan is that I will have hotkey triggers for a bunch of these, but they will be smart enough to do certain specific things, depending on the app, and the screen they're on. Slate cannot do this.
Next step is to control which monitor it's on without having to address the absolute coordinates, which would make the script more generic, and enable a 'max' width and height.