The Column Width helps to adjust the number of columns which an item occupies in total of 12 columns.
To adjust the width of a specific menu item, edit the Column width of it. You can find this option in the item editor on left panel
or hover on that item on preview panel and click on ↔ button
This item will have the width = exactly the column width you set.
When the total of items' columns in a row exceeds 12, the next item will be dropped to second row.
If all items set Column width as Automatic, they will be equally distributed on the total of 12 columns. (For example: if a menu has 3 submenu items, this menu will divide place 3 items in 3 columns equally as 4/12).
If an item has a fixed column width (eg. 3/12), but other menu items are set as "Automatic", they will auto adjust their width to fit the remaining space on a total of 12 columns. (For example: if a menu has 2 submenu items and you set the width of first item to 8/12, then the latter will auto adjust width to 4/12).
For image, you can set a fixed Image width, but this image width can't exceed column width.
Column width does not work if you enable Masonry feature. Read more about Masonry here.