The size of a chemical reaction is based on the energy of the reaction and the amount of materials used.
For example, lighting a match is quite small, but is a very vigorous reaction. There is not much material, so it is limited in size and stops quickly.
Burning a bonfire is a bigger reaction, but it is not quite as energetic as a match. However, it is much bigger because there is a LOT more material involved.
One of the largest and most violent chemical reactions was the “British Bang”. This was just after the end of the Second World War, when much of the left over ammunition, bombs and bullets was piled up on the island of Helgoland in the North Sea. It was then set off in a massive explosion.
However natural chemical reactions can also be huge. For example a forest fire can release huge amounts of energy, but it does go on for many days, so it is not an explosion. (It is still terrifying though!)
a good one is the sodium and water reaction! If you fill a flask with chlorine gas, and put a piece of sodium in it, then you add a drop of water, you get a a mini explosion and lots of yellow light!! Some street lighting is powered by this kind of reaction.
Or another one is adding a drop of concentrated sulfuric acid to a polystyrene cup of sugar but you have to do it near an open window because what happens is you get a mass of expanding black stinky sticky stuff growing out of the flask and it produces loads of black smoke too!