Have you seen full size billboards on buildings and along the roadways? How do they communicate their message? You may only be able to create large posters, but they can be very effective.
An effective poster usually has an image that will attract interest from your audience. The main written message needs to be large and only have a few words.
Posters need to be displayed in places where your audience visits all the time. Identify these places. You must have permission to display the posters. You will find that on some of the best places to put posters it can be illegal or against the policies of the organisation who manage the buildings.