Oops! left a line out of the instuctions Revised instructions below:
Go to Preferences> Tags(1)> custom tags
Define a custom tag with the display name 'Album bytes' and select 'Save to MusicBee database only'
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 'to tag:' to 'Album bytes'Tick check box in front of ALR preset so that it's calculated and saved at startup
Select 'Update preset' then 'Close'
You can now use/display the 'Album bytes' tag in MusicBee.
Now it should work for you