Originally Posted by DCB0

http://www.sheldonbrown.com/chainline.html
As Bill said, you select the spindle that the CRANK requires. What frame you have has no bearing on the spindle length, so Giant certainly will not give you any info.
FSA does list a spindle length for the double version of your crank. Perhaps you could use that number and assume the chainline has to be a few mm farther out and buy a BB accordingly. Remember that for a double, chainline is measured to the centre of the space between the rings, and with a triple is measured to the middle ring. Do the math. Get a spindle that is longer by twice the amount you need the rings to move out.