Is this the scarf you're talking about? There are others with the term "infinity" in them so I'm not sure.
http://www.redheart.com/free-patterns/infinity-scarf
You can make it any way you want..there are no knitting police.
It won't look like the picture though other than shape if you don't make it like the pattern. As for yarn.. The yarn called for is an Aran weight which is a heavy worsted/worsted. If you double the yarn you'll get a bigger scarf. It's okay to use worsted weight. If it needs to be longer just make it longer. Scarves are pretty forgiving.