To change the default font or text size for composing emails, please follow these steps:

1. Open your mail program, such as Outlook or Gmail.
2. Navigate to the Settings, Preferences, or Options menu.
3. Under the Formatting or Composing section of the options, locate the Default Font option.
4. Choose your preferred font type and size, then save your selections.

Once you have changed the default font and text size in your mail program, the selected font and text size should be applied for any emails you compose from that point forward.