Its not impossible and if you watch a decent amount of tv then doing something besides sitting there would be a good way to introduce exersize! The size of the ball depends on you height. You want to be able to sit on it with your knees and hips in 90 degree angles. Usually, women use a 55 cm and men use 65cm but again it depends on height. You can get them at most of the ''box'' stores like wal-mart for cheap. Even if you just sit on your ball to watch tv it will increase physical activity, but you can bounce around and move your hips around to work your torso. It's actually fun!

65cm is probably a good size for you, unless u are extremely tall.