What would you say to someone who believes their actions are too small to make a difference for the environment?

Small actions collectively and consistently taken in right direction will surely make an impact.

The action of one person is enough to begin a revolution, as can be seen throughout history.

Somewhat true. Banning ICE vehicles and pushing for EVs doesn't make the coal electricity more ecofriendly.

Even small things can add up to effect enormous changes

Everyone plays an important role in a community. And the combined efforts of individuals can have a huge impact on the environment.

No action is small. Every small action can make a large scale difference. If every individual can start small, a positive outcome will be inevitable. If no individual takes an initiative there will be no outcome.