Assuming 30 rows in ten abreast on a 777-300, as opposed to 9 abreast, that's an extra 30 seats, or about 3 more rows.
Much easier to either reduce the seat pitch and the leg room, or stretch the aircraft by three rows, whats that, er 3ft per row, so another 9 or 10 feet should do it. 10 feet is 120 inches, assuming standard 20 inch frames thats a stretch of 6 frames?
How much longer than a 777-300 is the 777X?