Can you use xdefaults to set the geometry of any window?
 

I'd like to have konqueror's "file already exist" dialog to show a bit larger so it has all the info shown rather than just an icon. I'm not using KDE itself (openbox) so KDE's own window rules settings don't apply. Can I use xdefaults to achieve that, or is it specific for terminal windows, as it seems to be judging from what I've found?

AFAIK each app sets its own; apps may or may not look in .Xdefaults or .XResources according to the app's creator's decision (or lack of!).