small / medium / large / jumbo
I like the idea of creating an extra size on the large end of the spectrum rather than the small end of the spectrum because of the difference in sizes.
32 / 48 / 64 have a difference of 16 between them and 64 / 96 have a difference of 32.
"Jumbo" is also a fun word and seems to connote inordinately large.
I used to work at a concession stand that had only one size of drink. Only the drink price was listed. People would often ask what size it was. I would say, "We only have one size: double extra jumbo!" poking fun at the crazy names and sizes of portions in movie theaters.