A:
In the newer versions of Squeak (2.7 and beyond), you can simply go to the "appearance..." menu and select "system fonts...", and choose a new font size. -- Doug Way
David Mitchell answered:
Or, use "TextStyle changeDefaultFontSizeBy: 1".
See TextStyle class>>changeDefaultFontSizeBy:
"This sample method recreates the default textStyle, with font 1 being a size larger than the smallest. It then initializes most references in the system as well, although most windows will have to be closed and reopened to get the effect."
For more font info, see Fonts in Squeak.