Gauge is not too important, just as long as it is not too loose, as you are going to lightly felt it anyway. Fiberfill for stuffing also required.
Any feltable yarn will do, and seeing as it is going to be felted anyway just use the needle size that is recommended. If you don’t want to felt it then use a smaller needle than recommended so that the gauge is a bit tighter and the stuffing won’t show through.