When using a Bunsen burner:
There are two ways one can adjust the size of the flame on a Bunsen burner:
The Bunsen burner’s flame is a combustion reaction between oxygen and methane, kick-started with the heat of a flint striker. Increasing the amount of methane and oxygen increases the size of the flame, done by rotating the gas valve in the right direction or opening the oxygen valve, respectively.
This is because more reactant is introduced into the reaction, increasing the rate of reaction. Decreasing either reactant slows the reaction, and makes the flame smaller.