Every single thing in Minecraft is made up of blocks and pixels, whether it's terrain, mobs, or your in-game character. Since everything is made up of quadrilaterals, creating a circle in the game ...

The larger the size of the circle, the more beautiful it looks! A size 1 circle in Minecraft is simply a block, which is square by the way. A size 2 circle is also a square of 2×2 blocks.