I took some time to scale down the mask for myself , 3 percent for each size until I had 20 mask sizes. I believe I will name the largest No 20 and smallest No 1 , if a larger size is needed later I would simply call it No 21 for example. If by some weird situation someone wants a new born sized mask, I suppose it could be called a Neg No 1 ,  or No 0 , Neg No 2 ext.  Looking at the options I don't think it will be an issue though.