Size column in Windows File explorer shows size(s) for files, however it doesn't show folder sizes for folders. You can view folder size in Windows using one of below options.
In File explorer, right click on folder for which you want to see folder size, and click on "Properties" in context menu. This will display folder properties dialog showing folder size in "Size" and "Size on disk" display field.
You can also check other details like folder created date, subfiles/subfolder count and folder attributes (like hidden and read-only) in folder properties dialog.
You can also hover over mouse on folder icon in File explorer to display hover-over tooltip with folder size.