You might also want to drop 2-4 stitches from the total cast on number since you won't have any seams. Consider, also, the stitch pattern count if there is one. I like to place markers where the side seams would be.
As for length, assuming we're talking about a sweater and it says for the 2/4 size to knit in pattern for 10 inches then start armhole decreases, but you want it 2" longer, just knit for another 2 inches before you start the armhole decreases. At that point, you'll switch back to kniting flat and do the front first, then the back.