There are different shapes of the knifty knitter looms. The smaller round o es make smaller round tubes, larger round one can make adult hats, the rectangular narrow one can do scarves. Seems to me it if you're going to knit this way a variety would be useful. Kind of like regular knitters having multiple needle sizes.
How much yarn is dependent on how long you want or how big, and what weight yarn you're using. If you go to google and type in "knifty knitter patterns" you'll see a bunch. Plus there are some looms at multiple sites. Here's one-