If you really have a LOT of yarn, you could double the yarn (hold two strands while knitting instead of the usual one) and still use a size 17 (assuming they're a circular, not straights). This would make your afghan go much, much faster than using a single strand.
Just a thought! I made an afghan for my DH that was fairly simple, using this method. It's a basketweave design. Basically I did a seed stitch border all the way around (you could just do garter stitch for the borders which is knit every row), and alternated knitting 8 stitches/purling 8 stitches in the middle (between borders) to make a basket weave design. I did 9 rows (you definitely need a row counter for this pattern, at least I did!) like that then I would switch to purl 8/knit 8(again for 9 rows).
I hope I didn't confuse you
I'm sure whatever you decide to make will look great!
Ok, I just realized CountryKitty sad pretty much what I did (but she said it better!