Can the default font size for "Email this Page" text be changed in Mountain Lion?
There are several ways to do that. 

The easiest way would be to install a Plugin which formats outgoing E-Mail and give you the option to change specific characteristics to you liking (Font, Size etc.). For that you should look into the following Plugins:


MessageFont


  "WYSIWYG for your outgoing emails If Outlook for Mac, Thunderbird and
  other email apps can set the default composition font1, why can’t
  Apple Mail? With MessageFont, you can! MessageFont lets you set Apple
  Mail’s default composition font for new rich text outgoing messages. [...]"

Universal Mailer


  "Universal Mailer is a Mail.app plugin that solves some issues when
  sending emails. It removes ATT00001.htm files, it correctly formats
  messages with attachments and inline images and offers an option to
  set the default font for outgoing messages. [...]"



Another way would be to change the default Font Size via the Terminal, f.e. in 24, open up the Terminal and type in:

defaults write com.apple.mail MinimumHTMLFontSize 24


That will change the Font to 24pt, but that includes the Page you want to forward from Safari if you choose 'Webpage'. If you just forward the Link everything should be fine.