So, after going back to references of the originals and getting out my trusty wooden ruler again, I revised my digital model.
What we see here is the original mesh I made my porcelain cup from, superimposed over the mesh made to the measurements of the original prop's photo (and estimated for a height)
Unfortunately, when I'm 3D printing in porcelain I have a maximum of 125mm x 125mm x 200mm, or somewhat smaller than my estimated dimensions.
I directly scaled that down to fit within the 125mm and came up with a mesh that should hold about 12oz. Then I stretched it vertically back to the original height and found that one should hold just shy of 16oz. At the full estimated original dimension it'll hold 19oz, maybe able to top it off to 20oz.
It might be time to armor myself up and head to Starbucks.