Currently, the turtledemo doesn't allow you to change font sizes of the demo code, and the default font size is really small.
I can work on creating a patch to fix this. Best option be to have control-mousewheel change size, as is standard in browsers, as well as Ctrl + and Ctrl - to change sizes (since laptops don't have a mousewheel).
If tk has a problem with that, a second choice would be a menu entry "Font size with choices such as the current size (9?), 10, 12, 14.
Also, update the Help Text of this new feature.