Existing implementation:

Following the same schema outlined for the Export dialog (see first post), here is an alternate metadata dialog:
Proposed implementation:

Region A: Dialog Behaviors: height does not resize when dialog resizes
Region B: Preview: height resizes only when user moves splitter
Region C: "Options" (Tab) Area: height resizes when dialog resizes
Region D: Dialog Control: height does not resize when dialog resizes
In this example the thumbnail height (in Region B) is controlled by the splitter. The thumbnail width continues to be controlled by the dialog width.
The Options tab has been removed and a new Options button (the gear icon) placed in Region A.
I think the Save and Close buttons (right of Region D) should be renamed "Write" and "Cancel", but my example does not show that.