At that level, bigger is better. It's easier to wear a 22" on your head than a 20"; you need the extra for knots etc.

A bandanna is substantially more useful if its diagonal is long enough to go all around any part of your body, so you can use it as a belt or a bandage or as a loin-cloth or a bra or a cloak. A 30" one will wrap a 42" waist but without leaving anything for a knot. I like the 35" ones from Jinglebobs. However, these are silk, which is significantly lighter and thinner than cotton. A 35" silk has about the same weight and bulk as a 22" cotton. I've never seen a 30" cotton and it might be too bulky for every day use.

I found their 42" ones too big to be practical. All that extra material just gets in the way. At some point it's probably better to have two smaller ones.