Some phone mail apps, in particular the mail app built into Android phones, resize the text in an email if they think it needs it. The same email can then look smaller there than it does in Gmail or on a computer, even though the font size is identical.
Every email sent from the mailer now carries the standard instruction that tells mail apps to render text at the size it was written, and a base text size is set on the message itself rather than only on the inner content. That is what stops the automatic shrinking.
You do not have to do anything. It applies to every email, including ones built from your own templates.
If text still looks wrong:
- Check the font size you set in the editor rather than raising it to compensate, since that will then look too large everywhere else.
- Send yourself a test and compare it in two apps.
- Tell support which phone and which mail app, because behaviour differs between them.