Whatever tool and image format were used here generated that 29K file, and are what determines the size of the image displayed. In short, it doesn't appear that this question is related to Mail nor even particularly to OS X — but rather to whatever tool was used. Locally, I use Acorn, but there are any number of other image-related tools that can be used to create or edit images. For smaller image sizes, use fewer colors, and — for the view size — select a smaller size within the particular tool you're using. 29K isn't all that bad for a logo, but a text-format sig will always win.