Email is content, it doesn't have fonts, sizes, colors - just characters. And there's no setting in the GMail app to change the size of the text you're sending (because there's no "size attribute" in the email specification).
If your friend is seeing email in a small size, tell your friend to check the settings on his/her email app. Some apps can change the size of the text they display. That has nothing to do with email, just with what's being displayed.
@Crashdamage:
Any app that can do that is sending HTML, not email. And anyone who writes an email app that can send HTML should be roasted over a small match until he's well done all over. If you want to send HTML you send it as a file attachment, not as the body of a text medium like email.