Enable the size column (View -> Columns -> Size), and then you can see the
size (approximated) for each message in the message list.
Or, get Allen Watson's free "Show Message Sizes" script from ScriptBuilders @
MacScripter.net. With that, just select one or more messages, and run the
script. You can choose to view the total size of all of the selected
messages, or view the sizes of each individual selected message.
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http://www.scriptbuilders.net/?search=show+message+sizes>