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What caliber?

It depends on what you want to hunt and how much you want to spend. .25 is not a bad choice. You can stretch your range out further while maintaining more energy. It increases your odds of getting a quick, humane kill on larger animals. If you only hunt small animals like birds and rabbits, then .22 would fit your needs. Another thing to consider is the pellet. H&N Baracuda Hunter Extreme pellets expand well and transfer a lot of energy into the target. These group well from my .22 Condor, but not as well out of my FX Bobcat .22 and Royale 500 (they love JSBs). I have found that both my .22 and .25 have comparable accuracy at 50 yds. My .22 Bobcat and .25 Royale 500 are capable of dime size groups at 50 yards. I would advise you to see if you can find a way to test out a couple of different calibers and rifles before you make a decision and invest your money.
 
I have a .177 Disco and it has nearly 100 confirmed squirrel kills so I've been very happy with that caliber's performance. However the .22 has better knockdown power, thus the ability to take out larger animals and better wind resistence but with a loopier trajectory, more expensive ammo cost and farther traveling shots (i.e. be careful where you shoot).
 
I agree with most of what has been said already about the .22 caliber, but AK73 hits the nail right on the head regarding .177: flatter trajectory, cheaper ammo, and will do the job if you do your part. Don't forget, you're looking at a PCP, not a springer. You can sling the really heavy .177s and not see much appreciable change in your trajectory (I think I saw 16 grain .177 pellets on the Pyramyd Air website the other day). I've smashed big woodchucks with my .177 Benji Trail by hitting them in the fuse box. I've also watched them stumble away injured as I fumbled to reload because I blew the shot (in .177 AND .22). A Disco is pretty powerful for an entry level PCP, and mine in .22 shoots pretty darned flat with the right pellet (and at reasonable ranges). But a .177 may yield a higher shot count, and could be more accurate at greater distances? Bigger is not automatically better, just scan youtube for some of the European guys hunting vids. They're doing "lights-out" one-shot kills on squirrel and rabbit with 12 foot pound .177 guns! They just use proper shot placement to do it.