Using classic Outlook for Windows in business environments
For classic Outlook for Windows, change the default font size for new outgoing messages like this:
- Press Alt+F, T to open Outlook Options.
- Go to Mail.
- Select Stationery and Fonts.
- Under Personal Stationery, choose the setting for new email messages.
- In the Font dialog, move to Size, enter the size wanted, and confirm with Enter.
- Select OK to save in Signatures and Stationery.
- Select OK again in Outlook Options.
If the text is only small while composing, but the sent message looks normal, the compose window zoom may be reduced. In that case, while composing, press Alt+H, F, S, type a larger size, and press Enter.
References: