Gent's if you are talking about pressure in lines or pipes etc, then pipe internal diameter has a massive effect. It is down to internal friction of the material on the internal walls of the pipe or tube. Bigger diameter less friction for the increase of fluid moved.
I work in an industry where amongst other things our equipment is used to supply paint around the car factories. Too small a pipe diameter and the pressure drops are horrendous. Resulting in too little paint pressure at the robot sprayer electrostatic atomisers.
If you want to compute the pressure diiference in pipes then take a look at the following:
http://www.flowmaster.com/solutionov...FQNQEgodHw--6g
Or failing that do a google on Pressure Drop Calculations.
Now one thing that does not change (if you alter the diameter of pipe work) is the amount of travel. So the travel will be the same it's just that you will be exerting less pressure if the pipework is too small.
Cheers Mike