Two strands of homespun?! That's gonna be a thick blanket! It's not the easiest yarn to knit with for anyone much less a beginner, but it should be very soft.
There are not guidelines or rules. You use the needle size that works for you. It's a bulky yarn so the 17's will probably be fine. Try a swatch with those and see if you like the fabric. If not try again with another size.
Are you using circular needles? You won't be able to make a blanket on straight needles..