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.
The best size for your postcard ultimately depends on where and how it will be delivered. Postcards sent to regular mailboxes can be as large as 6 inches by 11 inches or more. You could go as large as 13 inches by 17 inches, though this size won't fit in most standard mailboxes and will need to be folded.