The term 'BIG LETTER' can be solved by the crossword solution 'EPISTLE', which is a seven-letter word. An EPISTLE is a letter, usually written in a formal or didactic style, that is addressed to a particular person or group of people. Epistles were a common form of communication in the ancient world, and were often used to communicate religious teachings. In modern times, the term is still used to refer to any long, formal letter.