As big as a regular human being (woman)
If mermaids existed a common depiction shows them with hair and with breasts so they are mammals which, by default, would make them animals as well since mammals are animals.
If mermaids existed (which they don't) they would presumably be mammals. At least, most depictions of mermaids show them with obvious mammalian characteristics (specifically, obvious human characteristics; most mammals do not have large, permanent breasts).
We can grow big breasts by doing sex with other boys because they suck our breasts.
Most artwork depicting mermaids show them to have breasts. This would lead to the conclusion that without a doubt mermaids are mammals and therefore nurse their offspring. Generally it seems that they are modest about showing their breasts and wear a brassier of sorts made of seashells. While I would think that this would be very uncomfortable, this does appear to be their common practice. So just as dolphins and humans are mammals and the females of these species produce milk for their young, mermaids would do the same for their children.
BIG
as big as God intends
Big.
As Big as E
Big breasts are 'des gros seins' (familiar/slangish) in French.
Genetics.
It varies!