Mail signature changes font size when sent to Gmail
Here are the steps to make this work, even tried them on Mac OS X 10.7 Lion GM / Mail.app 5.0


Open Mail.app
Create a signature
Close Mail.app
Open Terminal and in Leopard/Snow Leopard go to ~/Library/Mail.app/Signatures, in Lion it's in ~/Library/Mail.app/V2/MailData/Signatures/
You'll find there a file with random digits such as: 4E725456-58C1-4FD7-8490-3048F994CC33.webarchive
Open that file in Safari, use View Source or Inspect to get the HTML content
Create a new file using your preferred editor, I used nano, and paste the html
Look for 'font-size = medium' and change it to the size you want, i changed it to 12px to match my Tahoma 12px mail font
(Optional) if you want to embed an image, call it using &lt;img&gt; to a remote, public location and remove all &lt;object&gt; &lt;/object&gt;
Save the file as .html
Open that .html file in safari and save as .webarchive
cp the .webarchive you created to the location of the original .webarchive file while keeping the correct name (replace it)
Open Mail.app and check that the signature is the way you want it...