Minecraft wiki entry for particle, associated text: Must be a three-dimensional coordinates with floating-point number elements. Accepts tilde and caret notations.
Specifies the size of the 3-dimensional cuboid volume to spawn particles in, centered on position <pos>, and multiplied by about 8 (using 1 1 1 specifies a cuboid of about 8×8×8 in size). Negative values may be used, but have the same effect as their positive counterparts (using -1 -1 -1 still equates to an 8×8×8 cuboid). Note that the generated particles aren't evenly distributed over the cuboid, instead it uses a Gaussian distribution resulting in a particle-gradient most dense at the center. Tilde and caret notation may be used, the resulting coordinates are then used as delta.