Entropy is the measure of disorder within a system. The second law of thermodynamics states that the amount of entropy will only increase overtime.
Therefore, in a combustion reaction entropy increases as the system is admitting heat and the molecules are more dispersed than it was before.
To measure the entropy of the combustion mathematically, you need to use Hess's law. To estimate the entropy, you can assume a few things. Most combustion reactions produce more gas molecules than they consume. In addition, all combustion reactions increase temperature of the molecules in the reaction. In both of these situations, entropy will increase for the system.