Who wouldn’t want to knit their very own Baby Groot? You can use pretty much whatever yarn you want- and base the needle size on what is suggested for the yarn you are using for the main body. For the arms and the “hair” I would suggest going down a size or two and switching to Double Pointed Needles. The whole pattern can be knit with Double Pointed Needles if you are not comfortable with the Magic Loop method.
The whole pattern can be worked on a 1/4 of a skein of Vanna’s Choice yarn- so you’ll have plenty of yarn left over to make more Baby Groots