I tried implementing something like this but unfortunately it turned out to be much trickier than I expected due to complications related to using dual monitors. I may revisit this later if I can figure out something that works reliably.
I'm a dual monitor user and your reply reminds me I ought to see about using the second one more with PWP. At the moment I don't so maybe I am missing some opportunities already.
Using a dual monitor definitely helps prevent the transformation dialog box from obscuring the images you are working on, but the issue of controlling the initial size of the dialog box remains.
Not specific to the issue at hand,
Just wanted to add that I also have dual monitors and have them set up so the main window is on one while the Image Browser takes up the second. Also when I open a Transformation/Dialogue Box, it too sits on the second monitor (in front of the Image Browser). This leaves pretty well the entire main monitor for the image.
Only the main monitor is color managed and the Browser is expanded to full monitor size (often have multiple image thumbnails across the screen).
Can't recommend a dual set-up more heartily.