Spur gear torque needed for a given pressure down on a shaft

[Tinkerer]

Hi, Was hoping someone here could help me with this. I want to push a shaft down at a given pressure- with a piston on the bottom say 1 1/2" dia. pushing 140 psi. I want to use a spur gear to push it down with a rack on the side of the shaft. I need to know how much torque would be required from the motor to accomplish this. The size of the spur gear would be the smaller the better. If I'm thinking correctly the smaller I go the less torque I'd need, but I get a bit mixed up there because I would gain leverage going bigger- but the movement would be less per RPM going smaller so I'm not sure. Let's say a 1" dia. spur gear, I could go bigger or smaller depending on the results. The motor would be geared way down moving very slowly just for the record.

Any help would be very much appreciated :)

[Tinkerer]

Just to clarify, I'm looking at motor/gearbox assemblies so the torque at the output shaft is what I'm needing to determine. Thank you.