Simple.
In MC for Audio files only for each Album, as an example;
Under the Audio group in the Navigation Pane at the left of Standard View, make a Category View with one Category of "Album", save that, then change the List Style to "Details".
If you just want to see the total in Excel, add a Column to the View for the field [File Size]. That will show values such as "295 MB total".
If you want to do calculations in the spreadsheet after collecting the data, add an Expression Column to the View and enter the expression [file size,0]. That will show the total size of all files for the Album in Bytes.
If you want to do calculations in the spreadsheet after collecting the data, but want the size in Megabytes (MB), then use the expression;
Math([file size,0] / (1024*1024))
If you want that size in MB, but rounded to a whole number, then use the expression;
Math(round([file size,0] / (1024*1024)))
See
https://wiki.jriver.com/index.php/Miscellaneous_Functions#Math for more options in rounding, truncation, etc.
Remove any Columns from the View that you don't want, or add any extras that you do want.
Select all items in the View (Ctrl-A).
Copy all items in the View (Ctrl-C)
Open your spreadsheet and select the cell that you wish to start the paste into. i.e. The top left corner. Paste the contents of the View into your spreadsheet (Ctrl-V)
Done.
If you actually wanted the folder size for all folders that audio files are in, for example if you have sub-folders for CD Discs as I do, then just make the Category View above for "Filename (path)" instead of "Album".
Once you have one or both of the above Views created you can sort and filter them as you desire. The Copy and Paste will use those settings, so that is what you will get in the spreadsheet.
The key to the above, other than the Copy and Paste functionality, is that the [File size] field will show the total of all files for the group of items being listed in a Category. No need to do any fancy processing.
All done in MC, so it works only on files imported into MC, and does not include ancillary files such as Cover Art, Extras, Sidecar files, or any other files in the folder. Just the files in the MC Library.
So if you want the total size of all files in a folder, including ancillary files, then use Wer's solution.