Janette, I, too, am about to start using Magic Loop, after doing a pair on dpn's
Oh, my! They turned out okay, except for a few beginner mistakes, but I don't see any reason for using dpn's again as long as there is a simpler method available.
I purchased Sarah Hauschka's book, 'The Magic Loop' and also found a great explanation for ML at www.knittingdaily.com
I also viewed Amy's video on ML, so maybe now, I'm ready
after all this preparation.
In her book, Sarah is said to prefer 40", but 32" is given as a minimum.
From Knit Picks, I ordered 32", but when I order more yarn from KP again, I may order a 40", too, just to see if it is a better length.
Hope this helps as you decide.