Force is pressure x area of the piston. Area is 22/7....3.14...... (pi, ratio of the circumference of a circle to the square of the distance from the center to one side known as the radius) times the radius squared (radius x radius).
Since the radius is half the distance across the circle measured at the center and is squared, a small increase in diameter results in a big increase in area and the bigger the difference in dimensions the bigger difference in the result, hence force.
So as a quick comparison, a 4" dia cylinder (2" radius) would use 2x2 whereas a 6" (3" radius) would use 3x3. You increased the diameter 50% but got 225% increase in force as BD stated.
So, either know what your item weighs, or get the largest diameter you can afford and reasonably use.....course it has to exceed the weight of the load if you plan on doing any lifting.....or up your hyd popoff and increase your pressure...but that's a linear increase. 500# increase on your current 1000 only gets you a 50% improvement but could solve the problem when added to the new cylinder.
I have noting to do but blab today so I sorta drug this out... Happens sometimes to old pfarts. Sometimes things are just better passed over. Grin