if the pattern calls for 100 stitches and a single strand of yarn and i choose to double the yarn how much, if at all, will that alter the final size of the blanket?
It can alter it quite a bit. However 100 stitches isn’t very many at all for a blanket considering I usually cast on 80 or so for a hat with worsted weight yarn.
it’s a small throw size blanket. should i plan for it to double it or somewhere in between original size and doubled?
plus it’s on size 17 needles
That’s different then - 100 sts with double stranded yarn will make something about 50" wide. You may want to CO about 15 sts first and knit a couple inches to see how many sts/inch you get and how many you actually need for whatever width you want. Sounds like maybe you only need 75 or 80 sts for a small throw.