If I need to maximize my yarn because I just have barely enough (for a scarf-not a fitted garment)…do I want to go up a needle size and make it “airier” to use less yarn? Because smaller needle will use the yarn faster, right?
Yep that’s right! 
Thanks! I just couldn’t seem to wrap my brain around the concept—but it seemed logical.
Make sure you use less stitches with the larger needle. If you use the same number of sts it may not turn out as long as you’d like, but wider than the original.
Yeah, I thought about this one for a while, too. But I think if you plan to do the exact same number of stitches, a larger needle will use more yarn. But if you are knitting by size (length in inches, etc.), you will require fewer stitches and less yarn to get there.
Yes, exactly. Knit to the same size, not using the same stitch number.