The process is quite intransparent, let me try and help you. First, you open this post as reference:
What you do (as far as I understand it and it's working flawlessly for me now) is the following:
You download the item under 'Download Everything Command-line Interface', this gives you the es.exe you need to then put into Program Files/Everything:
https://www.voidtools.com/downloads/#cli
You make sure Everything is running, I suggest you activate Everything as a service if you haven't already (you can use this method with either 1.4 or 1.5 alpha, you just have to make sure only one instance is running and you have alpha_instance=0 set in everything.ini.
Now you download the two text files Ixp attached above and put them in the posted folder 'Script AddIns'.
Then right click on DOpus title par and activate customize mode through 'Customize...'.
Righ-click on the toolbar and select either 'New -> New Button' or 'Insert New -> New Button' depending on where you click.
Create two different buttons. Right-click on each button and click 'Edit...', click 'Advanced...', pick 'Script Function' under 'Function', select 'JScript' under 'Script Type' and replace the text below with the content of the two green code boxes above. Then you can remove the buttons from the toolbar (I clicked them once, but I don't think you have to do that, it's just to add the code to DOpus' DB). Deactivate customize mode.
Now you can right-click on the columns and tick 'EverythingFolderSizeVars' under 'Columns -> Script' and voilà (I changed the label of EverythingFolderSizeVars, so the column isn't too wide). Now you can optionally add the XML buttons to toggle visibility and refresh or you just leave the column activated permanently.