To use xpExplosiaFX, you need a source object (something to burn), which must be located inside the solver boundary.
In addition, there must be an xpExplosiaFX Source tag on the source object.
xpExplosiaFX Solver tab.
This is the voxel size within the boundaries of the solver.
The solver volume is divided into cubes of this size.
Smaller values result in more accurate simulations but are slower to playback and render.
As the voxels get smaller, more particles are required to generate the smoke, etc.
Animation demonstrating Voxel Size settings of 3cm (on the left) and 1cm (on the right), within the solver.