Not perfect but,
For TextEdit, you will not be able to add size 16 to the drop down menu, but you can change the size in fonts & set the default size for TextEdit documents in Preferences, click on Change…
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Select the font size in the box highlighted below, & type the size you require:
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You will need to quit TextEdit, & re-open for this preference to be retained:
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Nb. This will only work for one font style, in Plain text, or Rich text. & though the size shows on the drop down menu box, you will not be able to revert to the custom default, if you change the size.
Sorry, I cannot offer any help for this in the Mail.app, as I have never used it.