Thanks for your reply.
I don't think think I made it clear has to what I needed.
I need to know how to find the actual pixel area the gallery will occupy, ie. 600x900px. not the megabyte capacity of the file.
I just looked at a couple of a galleries and it appears that they scale dynamically with the browser size-particularly items in the header. I am not sure, short of scouring source code, or using a screen ruler application that you can get this information.