Emails always start off in 9pt and I always have to F2 the text up a font size. I can't see a way of changing the initial font size in any settings though.
Font and font size settings are determined in the mail form in your mail file template. This is nothing that you can set anywhere.
Changes must be done with Notes Designer on the mail file template your organization uses - and then the changes are replicated to your mail file.
Contact your Notes Designer / admin - they would have to change this.
I no longer have access to r5 - that one's quite old! If memory serves it's something like:
File -> Preferences -> User preferences On the Basic tab, click the "Default font" button.
You'll need to restart Notes for the change to take effect, I think.
The later Lotus Notes preferences have a section just for email, but I can't remember if R5 has it or not.
Update: Other things to try:
An important note about editing your notes.ini:
This is a workaround, but it may be helpful if you have NO design access to your mail file, and your IT department won't help.
Notes has a "Permanent Pen" feature. On Notes 7 it's under
Set a permanent style to the font/color/size of your choice, then use the shortcut key to enable it when composing an email (On Notes 7 I press