If you're interested in calculating vacuum pressure vs. vacuum cup size vs. lifting capacity, there's some basic info
here.
Keep in mind that vacuum pickup cups are very sensitive to leaks - it doesn't take a big one to cause a loss of attachment. Also, if you use place the vacuum generator far from the cup(s), and use small-bore tubing, it will take a loooong time to attain or release vacuum out at the cup. Finally, if you have multiple cups connected to one vacuum generator, a leak at one cup will affect the others unless you play games with restrictions in the lines (which will slow down the vacuum flow even more).
My advise to anyone who hasn't used vacuum before: don't expect vacuum to behave anything like compressed air, and remember that a perfect vacuum is only 14.7 psi (and you'll never get perfection).