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If you're not showing the size of the emails yet, you should turn that on by going to View -> Columns -> Size
When the Size attribute is checked, the column for email size will appear in your main Apple Mail window view.
Once you're viewing set your emails to sort by largest emails first you can then save the large attachments and then delete the large emails you don't need. This strategy will give you the most bang for the buck in freeing up the most space in your email for the least amount of work.
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