#20 Disher/Scoop?

I am looking to purchase a disher/spring-loaded ice-cream scoop. I will use it mostly to make muffins, although probably also occasionally cookies. I know that #20 is the appropriate size disher for muffins (according to Alton Brown, anyway), but when I look online it seems that every disher for sale that is marked as #20 is a different size -- some are 1.5 oz, others are 2.5. I also found one posting where someone said it was 3.2 oz. Does anyone out there know what the correct size -- in ounces -- for a #20 disher is? Thanks!