A while back Rob Houweling ran into a textbox that automatically made the font smaller when the text was larger than the size of the textbox. He wanted to recreate one in Silverlight so he did it.
The textbox had to have a property for the minimum size of the font. We don’t want our users to see a textbox with a font of 0.25 pixels. That wouldn’t be a good user experience :)
The rest of the stuff has to go pretty much automatically. When the user types, the fontsize automatically adjust. It gets smaller when the text is getting larger than the available space in the textbox and the fontsize gets larger when there’s enough available space.