For pixel-lovers. 4x4(5) for best legibility in smallest size.
See more:
https://fontstruct.com/fontstructions/show/360418/nt3_00
https://fontstruct.com/fontstructions/show/1215601/fs_3x3_0
https://fontstruct.com/fontstructions/show/1406220/pixel-3x5
https://fontstruct.com/fontstructions/show/1340150/pixel4x4
https://fontstruct.com/fontstructions/show/589994/yet_another_small_font
http://ru.fontzzz.com/font/category/38_pixel__bitmap.htm
https://www.myfonts.com/fonts/vic-fieger/sixpak/
https://www.behance.net/gallery/33669138/Arapix
https://fontstruct.com/fontstructions/show/15399/thepix
This is based off of the exponent forms of characters in a TI-84 Plus Graphing Calculator from Texas Instruments. ~ is a square root symbol, & is a bar continuing the square root, ` is i (imaginary), # is pi, and $ is an arrow (like 2.5 >Frac = 5/2).