How can I add a font size to drop down menu??

A simpler solution would be to click onto the size, showing on the box for the drop down,

& type the size you require:

Nb. Apologies, you would be able to revert to the custom default, if you changed it.

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…

Select the font size in the box highlighted below, & type the size you require:

You will need to quit TextEdit, & re-open for this preference to be retained:

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.

The screenshots below are from TextEdit on Sierra, Mail is the same.

Select Format - Fonts - Show Fonts (Command T key shortcut)

Click the gear wheel icon at top left of the fonts window & choose Edit Sizes...

Add new size(s) & adjust the maximum/minimum shown : then click Done

The new size(s) show immediately in the Fonts window selection, you might have to quit/re-open the app for the main window font drop-down to change; although selecting the new size from Fonts window seems to prompt the change right away.

and then after quitting TextEdit & opening it again :

Could not find out font size 16 both in mail application and text edit for Mac OS High Sierra 10.13 .1

It was a very minute settings , thanks balsa 2 .