The font size of SmileBASIC has been fixed in 8x8.
You can make 16x16 fonts with following two methods.
#1: Use "WIDTH 16" to enlarge text fonts.
#2: Apply sprites or BGs as Fonts.
When you type letters on the keyboard or use PRINT statement, the letters will appear on text plane.
Actually, character size of text plane is fixed in 8x8 and you cannot change it to 16x16.
The method I can recommend is to use BG plane or Sprite plane for putting letters instead of text plane. And I think BG is better for it than Sprite, because longer sentence wastes more sprites.
Could you refer to the attached screenshot?
I've made a command "BGPRINT" to print any sentence (but capital letters only) on any location of BG plane.