Installed the thing, but i don't really understand what I'm supposed to do with the library report units now?
Go into Auto Library Reports (ALR) and select new preset.
Highlight <Empty preset>
'Functions for new fields:' should display <Grouping>. If it doesn't, change it so it does.
From LH column select <Album unique Id> then Album
Change 'Functions for new fields:' to Sum
Again from LH column select Size (not in the alphabetical list, it's about 3rd last)
Select 'Preview' and see if report compiles.
Now change 'Save field' to 'Sum(Size)' and 'Assign virtual tag function id:' to 'AlbumSize'
Select 'Update preset' then 'Close'
Open Preferences> Tags(1)> Define New Tags...
In the bottom Virtual tags section create a tag 'Album Size MB' with the formula:
'Apply' and 'Save'
You can now use/display the Album Size MB tag in MusicBee.