Not really version specific, same behaviour since 3.x through current 184.108.40.206.21:
When I right-click on a record in the result grid and select Single Record view, it usually comes up so narrow that nothing can be seen but column names. I need to resize and re-position the window to make it usable. It stays that way during the work session till I close SQL Developer, however when I restart the program, it forgets everything and I can start from scratch again.
Is there a way to ask it to remember the position and size of this pop-up window?
I don't think so. As you note, this has been the behavior for a long, long time.
Although the ide.properties file in the user settings folder may store some size information on the single record view panel, I believe our code ignores it and uses an algorithm instead to initialize that panel. The longer the column names, the less space is initially available for the column values.
The fact ide.properties stores anything at all for it must be an artifact of SQL Developer being based on the JDeveloper IDE.
SQL Developer Team