How large can I make an image that *DISPLAY would load onto the screen? I was thinking of making a scrolling background for a game.
I don't think there's a fixed limit, it would depend on things like amount of memory, how fast you need it to load and so on (version of Windows might be an issue too).
You also need to take into account how you propose to do the scrolling. If your intention is to *DISPLAY the image just once, and then scroll it using the @ox% and @oy% variables, the default size limit is 1920x1440 pixels in BB4W (bigger in BBCSDL). That can be increased but at the cost of complexity. Scrolling this way is very fast.
If however you plan to do the scrolling by repeating the *DISPLAY with different origin coordinates, there's no particular size limit but it won't be anything like as fast as the previous method.
Any seriously large scrolling background would need to be implemented as tiles for performance reasons in any case.