print #handle, "!font fontName pointsize" ;
This command sets the font of the text window to the specified name and size.  If an exact match cannot be found, then Just BASIC will try to match as closely as possible, with size taking precedence over name in the match.  Note that a font with more than one word in its name is specified by joining each word with an underscore _ character.  For example, the font Times New Roman becomes Times_New_Roman, and the font Courier New becomes Courier_New.

Example:
print #handle, "!font Times_New_Roman 10";

For more on specifying fonts, see "font"