That did the trick.

I defined a new view definition called
"My Attachment Size" using your suggestion.

Is there a way to apply "My Attachment Size"
View | Arrange By | Current View | My Attachment Size
to every subfolder of the Inbox so when I select a subfolder
"My Attachment Size" is the default view?
It looks like one must manually set the view to "My Attachment Size"
for each subfolder.

I also noticed that if I look at the properties for the Inbox,
there is a Folder Size button which gives the size of the
contents of each subfolder.

Thank you for the feedback.