I'm sure I've seen somebody do it but I can't remember how. 3 blocks on the horizontal (x) axis, 5 blocks on the vertical (y) axis, I need a letter 'M' I have a feeling it's impossible bu I think I've seen it. I tried but it looked like a 'H'.
Make it like the M in the achievements. straight line then the v shape in the middle then another straight line. With blocks.
Here is a Minecraft font picture for all the Minecraft letters: I dont think an "M" would be possible in 3 horizontal blocks as an "M" has 3 "legs".