Use WIDTH 40 for big text. It changes the entire screen to wide text mode. Use WIDTH 80 to go back to normal size text.

 SCREEN 0
 WIDTH 40
 CLS
 PRINT "Wow! This is big!"
 INPUT "Press <Enter> to go back", A$
 WIDTH 80
 PRINT "That's more like it."

Unfortunately, you won't see the big text in a window. You'll have to press <Alt>-<Enter> to switch to full-screen mode. Be sure to press <Alt>-<Enter> again to switch back to window mode.