We are specifying 84" x 144" sluice gates in one of our projects and we expect very high torque and thrust required for the actuator to pull up the gate under a head of 40ft water.
The height of the cylinder actuator (either pneumatic or hydraulic)  will be within inches of the height of the electric actuator.
With the gate valve open, the lead screw will have to pass through the electric actuator and stick up into the air. (probably inside a fixed protection tube)
So you can have a 144" stroke cylinder, or a 144" long ACME- threaded actuating screw. 
A pneumatic cylinder will presumably run on your plant air system, so typically that means you can have 80 psi you can count on. 
A hydraulic actuator typically draws from a 1500 psi system,
So the relative areas of the pneumatic vs hydraulic are almost 20:1 (or a 4.3:1 diameter ratio, neglecting the (important) cross-sectional area of the actuating rod)
If you need fail action, you can use a pneumatic accumulator about the size of a tank car, or a hydraulic accumulator which would be much smaller, or you could use an electrical failsafe which would be basically batteries and an inverter, and comparatively quite compact. 

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