Okay, so this is weird - the Delphi reference materials
suggest that resizing is always possible; a "resizable" option (ofEnableSizing) exists, but supposedly does nothing; however, when I added it to the dialog's options flags, it became resizable (but aside from that, it remained as it currently is in 12.6.2).
"Resizable" is acheived in commit B3751D7, but I'd like to if possible fully restore them to how they were in the XE6 builds, so leaving this as an open topic for now.