The intended use of the map is one of the factors that determine the dimensions of a map. The area of interest needs to be shown at a usable scale. The printed area needs to be big enough for photos, or any other desired graphics. The size and shape of the area are too important to the scale and legibility of a map to rely on standard sizes. Paper sheet sizes can be ordered specifically for individual jobs to keep waste and cost down.
What is more important to you, the text and graphics, or the map?