4x3. Keyboard characters only (plus a very few extras). the @ symbol is a solid 4x3 block, which actually works fairly well. all lowercase letters, including p,g,q, (but not including the tall ones: t,i,d,f,h,j,k,l,b) are 3x3; and all are well-distinguished from upper-case. I don't think it's possible to make characters smaller than this... well, without limiting oneself to just uppercase or lowercase.
This is a cloneStrict monospaced 3x5 font. Reasonably legible and balanced. Includes upper- and lower-case, digits, punctuation, the whole lower half of CP437. Largely derived from Tom Thumb (MIT or CC-BY 3.0 or CC0 license): http://robey.lag.net/2010/01/23/tiny-monospace-font.html