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.