Width dropselect menu

How can I make a dropselect menu wider than the widest menu-item?
In the settings I tried to edit the dimensions (margin), but it has no effect at all for the width of the dropselect menu.

I found a workaround to add a header with a line, wider than the widest menu-item, but that isn't a nice solution.

See attachments

Hi
Please give me the url of your page so that I can check
CEd

For example: genealogiegroningen.nl/diversen/weblinks

The width of the dropselect menus below become equal to the width of the widest menu-item.
By placing the line below (as header) I can make the column wider, but it could to be done in the settings.

The dropselect menu doesn't take the settings into account anyway. When it should follow the settings the background colour should be blue, like the menus on the upside of the screen.

I've solved this problem by changing the dropselect menus to default menus.
Now the colours and dimensions of the menus are okay!