When we view a sign, the first time, our eye automatically goes to the center of interest. That's the dominant area. A good example of how this basic rule is violated is when there are several lines of copy, all the same size. Not only is it visually boring but the message is confused.
There are lots of ways to achieve this design element. Size and or color lead they way. Bigger isn't always the answer. For example, a large white panel with a small logo or copy located in the dead center can achieve the power of dominance through the use of negative space. The blank spaces, between lines of copy, or around it can be as powerful as huge letters.
http://en.wikipedia.org/wiki/Design_...nce.2Femphasis
I often see signs designed without understanding positive and negative space management. One of the most powerful logo's using negative space is APPLE. This comes under the rubric of "Less is more". There are also times when a color can be more important than larger copy. So size doesn't always guarantee dominance.
The topic heading should be the first consideration. If you're selling shoes or flowers, etc. that should take preference over anything else. It's amazing how often your eye isn't directed to topic heading. There's lots of power in understanding dominance.