Yes, there is a solution I just found. Do exactly the following:
- Go to any computer with Microsoft Outlook.
- Create a signature in Microsoft Outlook. Write your signature, edit the fonts, sizes, etc...
- Make sure the font and font size is the same as the one you set in Mail as your default font.(example: Arial 12)
- Save it, and send an email from Outlook with this signature to yourself (the computer where you use Mail)
- Once you've received this email in Mail, select and "copy" the signature
- Go to Mail, and create a new signature
- Paste the signature (the one you sent to yourself from Outlook)
- Make sure that "Use the same message format as the original message." is unchecked !!
Now try to send an email with this signature, open the email in any mail client like Gmail, Yahoo, Outlook, etc... and now the signature should have the same size as the text.
Warning: whenever you want to modify your signature, you have to re-do this entire process. Let's say you want to put a word in Bold in Mail Signature, do not do it, it will screw up everything again. So make sure when you edit your signature in Outlook that it is complete and properly edited.
I know it's not easy, but Apple seems unwilling to solve that MAJOR issue....