friends receiving my mail in small fonts

Are you using the Bo Derek font?

Mail doesn't send email in the default font you set in preferences. That is just the font that is displayed when you are composing. If you want to set a specific font or size, set that when you begin composing the message.

You can't control what the other end does with your email. Their email program displays the email however it likes.

When you start the email, do you change the font and the size, or are you changing those settings in the preferences.

Others have complained about the tiny fonts from me, too. A good technical reason would be helpful about now.

I just sent a test message to a friend with a Yahoo account.

With the new message window open, I chose Format > Show Fonts > Helvetica 36.

This is what was received at the Yahoo account:

Setting it in Preferences will not work - you need to do it while the message compose window is active/open.