How big is a baby's stomach?
The first 30 days involve rapid growth, but generally, fall within a standard proximal range of 5 - 7mL on day one. By day 3 the stomach has expanded vastly to nearly 23 - 27 mL. From this point on, growth and size to age ratio will vary a bit. 

https://themomivist.com/2016/10/10/the-newborn-stomach-size-myth-it-is-not-5-7ml/

Here's a link to an article that has some efficacy. Many hospitals and physicians use different techniques and believe differently on this particular subject. Considering the laws surrounding experimentation on fetus and children, the "Size" of the stomach debate only has efficacy if we were able to case study THOUSANDS of infant stomachs which isn't likely to happen anytime soon. I hope the link helps and maybe points you in the right direction. There really is no 100% correct way to answer this as the studies surrounding it just don't have any weight to them.