Anyway, there is a setting in Nautilus that automatically enables showing files sizes in both desktop and in the file-manager itself which is quite useful most of the time. Although even without enabling this you can get the "size" of files the more you zoom in (more than 100%), yet I'd like it to be there by default. This is how you can enable this option in Nautilus.
Open Nautilus and from the menu go to, "Edit" -> "Preferences" and from that menu choose "display" tab. Once you click on the "display" tab, it'll bring another window and from that window under the "icon captions" sub heading, click on the first button which says "none" and change it to "size".
Then change the button below it to something other than the "size" which is its default. After doing the changes close the window and when under the default zooming level (100%) this will make Nautilus (including on the desktop as well) show file sizes by default!.