Gauge is everything with this sweater. I would try a chunky yarn and 6 mm/6.5 mm needles, but there's no telling what you will end up with until you try. I'm sure there's a formula, but I don't know what it is.
As for how to lengthen, what I did was pick up stitches at the end of each arm and did more garter stitch for a couple of rows. I then decreased some stitches and continued in garter stitch. (I think I went from 45 to 34 stitches, though I can't really remember) ... the decreases gave a tapered effect.
Good luck, I just love this sweater.
It's quick and fun and so amazingly engineered.