Subjects tell the listener and the reader whom or what the sentence is about. The subject is that part of a sentence which names a person, thing, or idea. e.g. Every complete subject contains a simple subject. The simple subject, which is a noun or pronoun, is the most important word in the complete subject.
The USPS considers a postcard to be any single rectangular piece of paper that is at least 3 1/2 inches high by 5 inches long by 0.007 inch thick' and no more than 4-1/4 inches high by 6 inches long by 0.016 inches thick.. The post office considers all papers outside of these dimensions to be letters.
With USPS Marketing Mail, there is a little more flexibility -- there is no separate (lower) price for postcards, so you don't have to worry about your postcard being too big -- because you're paying letter prices anyway. But make sure that your postcard is no larger than 6-1/8 x 11-1/2 x 1/4 thick.