A 40" is pretty long and you can't do smaller things with it except for flat knitting, so I'd suggest something about 32" long. You can make hats with that if you use the magic or single loop techniques.
The scarf you linked to looks like bulky yarn and the 10mm needles should do just fine, 12 mm is pretty thick. The pattern looks like seed stitch.
Yes, you can do an infinity scarf with straight needles and sew the ends together. You would do that with yarn and a tapestry type needle, or you can do a provisional cast on (a temporary one you take out) and either graft the ends together or do a 3 needle BO.
sue- knitting heretic