Not sure why it is like this - all the openings seem to have different minimum values.
You can press the 'Empty Opening' button at the bottom left of the settings dialogue.
You will lose some of the parameter options but that probably doesn't matter if it is just a plain opening.
At least there will be no minimum size.
Barry.
One of the forum moderators.
Versions 6.5 to 27
i7-10700 @ 2.9Ghz, 32GB ram, GeForce RTX 2060 (6GB), Windows 10
Lenovo Thinkpad - i7-1270P 2.20 GHz, 32GB RAM, Nvidia T550, Windows 11