5 inches more, thats all you recieve, I don't know how much more it is, but if you fall under the following:
Economy Plus is available on a first come, first serve basis to our best customers including:
Mileage Plus® Elite members (Premier®, Premier Executive®, 1K®)
Qualifying United Economy® customers (Y, B, M, E and U fares)
United Global ServicesSM members
Star Alliance® Gold and Silver members
then be at the airport first, or try calling 24 hours in advance, I don't know if they would give it to you or not.
Anyways, it says your between 13-15 (same here) so i don't know how tall you are, so it comes down to whether or not your knees want the extra bit