The 737-500 Shuttle is mainly used for flights to medium size markets from the four hubs in San Francisco, Denver, Chicago O'Hare and Washington Dulles.
There are two different configurations. A little more than half of the planes are equipped with Economy Plus. But these ones are former Shuttle aircraft and offer significantly less service. While you can find out what version United plans to use for your flight, these plans do often change even a few hours before departure.
Acceptable First Class seats with limited legroom
Seats 2A, 2D
Fairly good First Class seats.
Sitze 2B, 2C
Fairly good First Class seats. Your legroom is reduced a little by a metal frame under the seat in front of you.
Row 3 offers really good legroom, particularly on the starboard side.
Regular Economy seats with a pitch of about 30-31 inches.
Seats 20D, E, F
These seats have limited recline.
These sats offer a little more room and a two-seat row appreciated by many couples, but you do sit right in between the galley and the lavatory, which can be annoying. Decide for youself.