TLDR: a post in which I offer miniature makers/printers some useful scale guides.
Miniature figures: does scale really matter? Nah, not really ... also, of course it does. It depends on how tactically your home game is represented at the table and played. I have never really been that concerned about the scale of figures in play, but once I started messing around with 3-D printing that all changed because I started picking up STL files with no idea of the scale in which they were designed, which left me uncertain as to how to shrink or grow them, if at all, in my slicing software of choice (Chitubox as of right now).
So, I set about trying to figure out how to translate between "real life" (e.g. fictional world) measurements in feet and miniature measurements in inches and millimeters. The most common scale is something like 1" = 25mm = 3' of fictional height. Most humanoid figures at this scale are around 2" (6' fictionally) tall. But it gets more complicated because most figures take up one space (1" or 5') on a battle mat. Confusing? Yes, because 1" is both 3' in height but 5' in diameter – except it isn't. The difference is that 5' is "fighting space" not that the figure is literally 5' wide. In other words, base sizes are an approximation of how much space a creature needs in which to operate. From a mathematical standpoint, for figuring scale, base size is an afterthought. It's best to work everything else out and then choose a base size that makes sense around 1/2 of the figure height, usually.
If that's clear, then let's move on to fictional size categories in D&D. I'm not sure when this started (probably 3e but I didn't do the research), but D&D has some fairly clear categories of size from Tiny (less than 1' tall) to Gargantuan (20' tall or more). D&D also has a guide for how many spaces each of these size categories needs on the battle mat. Excellent. That makes things relatively easy.
Note that I say "tall" but sometimes the greatest dimension of a figure is length, e.g. a giant crocodile is not very tall but awfully long. I would suggest thinking in terms of greatest dimension.
Given all that, here's how the math works out for each foot of fictional height in 25mm and in 28mm.
Basically my thought process when approaching a 3D print of a mini is this.
1) Determine or decide how tall/long/wide the creature would be in the fictional world. Sometimes the answer is spelled out in a monster manual.
2) Translate that into rough mm dimensions and scale the model to suit.
3) If the base is integral, see how close you are to one of the "stops" in the chart below and adjust if desired. If separate, print the base you need.
Basilisk are 10' long in 1981 Moldvay/Cook D&D. 10' is roughly 3" or 80mm in length and straddles the line between Med. and Large. In the 5e Monster Manual it is listed as Medium. Ok, what do I do with that? If I print it 3" long it will probably need a 2" base (a lot of the figure being its tail). I could either find a mini with the tail wrapped around or I could just shrink the creature down so that it looks right on a 1" base. The best answer is probably a 1" base with the creature sticking out into a second space on the battle mat. But I could easily see a 2" diameter base or a 1x2" base.
It's not always easy. :)
I hope this helps someone!