Can I add "16 pt" option to font pulldown menus?
It's a flaky problem, or at least the solutions are flaky. As such, some apps, e.g. Mail, go through the motions but don't actually add the new size. Additionally, it may vanish at some undetermined time in the future. But for the present….

From within the app (e.g. TextEdit) that you want to see 16 pts, pull up the font dialogue (Cmd-T). In the upper-left corner, you should see a dropdown button with a gear icon on it. Click that and near the bottom select 'Edit Sizes…'.





In the 'New Size' field, type '16' and then click the '+' button below it. Click done. Quit and re-launch the app.