This hat is incredibly simple to knit, and as the sizing is done by taking a little double pleat into the hatband after knitting, there is really no need to swatch or measure unless you know you always knit at an unusual tension. Just knit it up using any aran weight yarn that knits nicely on 5mm needles, and it should work. My hat used 94g of a very typical aran weight wool, so you should get this out of a 100g ball without any difficulty. A couple of buttons or other embellishments add the finishing touch.