Building block letters in Minecraft is straight forward with this generator. Complete layout guide and multiple font sizes, with and without stairs. For large block letters you can choose almost any block. For smaller 3x3 Minecraft block letters you will need to choose a block that has stairs and slabs.
To build Minecraft letters you will need a range of blocks, stairs, and slabs so for this purpose included a convenient block counts so you can build it all in one trip.
If you're not looking for a generator here's a quick snapshot of all the 3x3 optimized block letters
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addedZoom/scale block size
addedMinecraft style font, with stairs and slabs
addedPixel text generator, with 3 fonts, basic styling