Rate of a reaction is the speed which a reaction takes place.
Rate of a reaction depends on the surface area of the particles. More is the surface area, more will the rate of reaction as more of the particles surface would be exposed for the reaction to occur.
Surface area is increased by decreasing the size of the particles.
Thus a large log will burn slower as compared to the same log cut into small sticks of wood as lesser area will be exposed to air and thus rate of burning will be slow.
