Mac mail : how to increase font size ?

Hello everyone 🙂

Since I've updated my MBA to OS Mavericks, I faced pretty annoying problem viewing some messages in mail.app :

Prior to 10.9, the following command-line command used to work:

defaults write com.apple.mail MinimumHTMLFontSize 12

But in Mavericks, it seems this no longer works.

Anyone found another way to increase the default minimum font size for new messages? Some HTML messages are coming in with tiny, almost unreadable font sizes.

Thank you for your time !