Little bit of interesting history: according to the American Presidency Project at the UC Santa Barbara, President Obama's State of the Union addresses to Congress are, on average, 7,080 words. Only one president has had a higher spoken average: Bill Clinton, at 7,426 words.
However, before President Woodrow Wilson, with the exceptions of Presidents George Washington and John Adams, the State of the Union was primarily delivered in written form. Of written-SOTU presidents, William Taft's portly average of 22,614 words per SOTU eclipses all but his two immediate predecessors, Theodore Roosevelt (19,656) and William McKinley (18,578), as well as James Polk (18,014).
The awards for concise average SOTU addresses go to President John Adams (1,790, spoken) and President Thomas Jefferson (2,578, written). The longest addresses of all time were President Clinton's 1995 address (9,190 spoken words) and President Jimmy Carter's 1981 epic 33,667-word written megalith. That's more than 100 pages, but I guess when your term's about to expire, you might have some extra lame-duck time on your hands.