Go to Tools/Options/Message reader and check the box 'Use alternate font for printing' and set it to the right size.

This assumes you are using a more recent (than 94) version of the program !

I feel foolish. It is there

The boss has a high resolution graphics. He adjusts the font so he can "easily" read messages on the screen. However, when he prints the message, it is way to large. Is there any way to have the preview screen use one font definition and when printing use another font definition? 

I have been using Pegasus Mail since 94, and I don't remember any place where I can change the message font that does not apply to both the "display" and "print" version of a message.