When designing an 24″ x 36″ poster, create your document ¼” (.25″) larger. This can be done easily in most Adobe Creative Suite programs by either increasing the height and width (like in Photoshop) or by adding a .125″ bleed on all four sides of your poster. This will result in a final poster size of 24.25″ x 36.25″ or 7,275 x 10,875 pixels at a resolution of 300.