Pneumatic Cylinders

[cosworth34677]

ok im about to make a list of parts for my excavator. over the next weeks i think im goingto try to convert it to hydraulic. but as far as the cyclinders i read that folks use Pneumatic Cylinders. so my question is ...

what size cylinder should i use? i order the hrc pump and valves . i know folks said not to but i got just as many folks saying too, to. so i'm taking the dive besides if it sucks i can take it apart and try to make my own. my excatator uses 2 60mm firgelli actuators and two 100 mm actuators. but looking on ebay the mass number of different sizes and bore and strokes single and double action is just making me confused. so help guys please!!