What you need:

12mm deep socket and 12mm wrench
14mm deep socket
14mm wrench (if you are installing ES endlinks)
New, larger D-bushings
High-temp grease

how to:
Jack up the rear of the car, both sides
Undo the bolts by using a 12mm socket on the bottom and a 12mm wrench on the top on both sides of the car
Undo the D-bushing mounts (in the center) that attach to the subframe with a 14mm socket
Pull off old bar
Lube new D-bushings and put them on the bar
Replace the D-bushing mounts
Mount the bar to the subframe where the D-bushings mount
Tighten down each side
Tighten down the middle (D-bushings mounts)
TORQUE DOWN on the sides if you have the new ES endlinks. Be careful if you have the original endlinks, they are known to break
Torque down some more
Torque down some MORE
Lower the car
Drive, and enjoy how the rear seems to "follow" the front more

Now as far as the bigger has in performance, i figure bigger = more stiffer. but for everyday driving too stiff may be dangerouse.
91-96 Escort LX= 19mm
91-96 Escort GT= 21mm
91-96 Escort Wagon= 23mm Hollow

I currently have the 21mm bar, have had it for 6 months now, no problems, pretty good and stiff rear end. Another thing that is good is upgrading the Bushings and endlinks. Energy suspention and Prithan offer Poly-urethan bushings and endlinks.