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Is it possible to compose an email in Gmail with an 11pt or 12pt font size?
(In a way that the recipient also sees the font size difference.)
As of Feb 2015, the only options available are:
Would love to know if someone is aware of a plugin, browser extension or workaround to enable this?
It is now possible to label and/or "star" messages when you send them.
Official Gmail Blog: Star and label messages before you send them
When composing a new message, you can assign labels or star it by using the labels drop-down menu. As you’d expect, recipients of messages organized this way won’t see your labels or stars.
This is the new compose window:
Press the down arrow in the lower right, then choose "Label".
This is the old compose window:
This is not possible at the moment. If you look at the list of search operators, you'll see that the closest thing you can do is filter out all emails with an attachment.
An alternative would be to use an IMAP client (such as Thunderbird) and then use that interface.
Since you want that feature, express your needs over at the feature-suggestion site of Gmail.
Update 2014-02-20: Gmail has now size: and larger: operators.
Best Answer
Google doesn't allow you to write emails in HTML. You can either use the Outlook website to compose the email and forward it your Gmail account and copy-paste the rich text of this message in other messages.
Alternatively, you can use the ctrlq.org/html-mail - it uses the Gmail API to send HTML mails from your Gmail account.