Get an Interlisp Exec (REPL)
Click and hold the right mouse button on the background, hover over EXEC, move to the right, go to “Interlisp”, release the right mouse button.
Increase the font size
In an Interlisp Exec, type one of the following:
(FONTSET 'BIGGER) (FONTSET 'HUGE)
New windows and future lines in the Exec will now be in a larger font.
Edit a function definition
To edit the definition of a function FOO, type into an Interlisp Exec:
This will let you draw out a new SEdit window, which you can use to edit the code.
Middle-click in the titlebar to get additional options.