I am starting a lap blanket that calls for US8 size needles, cast on stitches 180. I am seeing needes US8 16" and 24". Which one should I buy (16 or 24) ? Secondly, can I use either one for smaller patterns in the future as long as it calles for US8 size ?
The 16" are mostly for hats, 24" can be used for a lot of things including a lap blanket, and even a hat if you use a version of the magic loop technique. But 29 or 32" ones are a pretty versatile size, able to be used for small items or larger. Buy what's most readily available to you.