I disagree with #3. Shortening the chain to the point you can't use big/big, is like taking your car and at 60MPH shifting it into reverse it's a good way of leaving pieces of transmission (in either case) all over the road.

The shop that put on the crank though, may not have shortened the chain enough. Essentially when sizing a chain, you want it to just fit the big/big combination, there are probably 65,536 pages on the Internet on determining chain length, google is your friend. What I do is feed the chain through the RD and FD, with them shifted to the big/big, pull it as tight as possible and cut the chain at the next full link, if you do end up in that combination, the drive train will not self destruct.

With the chain just able to fit big/big, if there is slack at the small/small you need a longer dérailleur on the back. Although neither big/big or small/small should ever be used, nearly every rider at some point will end up in either one. What I do, is take a small container, a 35mm film can is really good for this, and put the extra links that I take off in there, then if I damage a chain, I have those extra links to repair it. Say you bust a link, you take out the damaged links and replace them with the spare ones from that chain, your back in business. BTW if you do bust a chain, it's probably time to review your bicycle maintenance procedures, as properly maintained chains rarely break.