Having spent a few years here I have started to wonder. "Why are these tags such a mess?"
The idea with a tag is that implies a certain content to be found in the image. But not every tag is a simple objective yes or no case to tag. So I would like to suggest we improve these somewhat vague tags and how/when to use them.
Personally I have found the "macro" and "micro" tags to be fairly pointless at the moment.
The reason why is due to it frankly being misused, and honestly there has been a trend towards just compressing everything "large" into the macro tag. For an example, "giant" is just an alias of "macro" even if it were its own tag before with a fairly standardized size range. (A range from noticeably large up to the proportions of a building. But beyond macro were instead used. Now it is all just macro....)
The giga tag is likely to suffer the same horrid fate to be fair. But at least it has a more obvious threshold compared to macro. (since giga is any character larger than a city and beyond, and where there is clear indications of such in the image.) Nano is the obverse of giga.
How the practices currently stands is as follows:
Macro is currently used for any character that is noticeably larger than another character in the image, and up. Ie, if the "macro" isn't bigger than a grain of rice is apparently fine, as long as there is someone else in the image that is considerably smaller. (Here both characters should obviously be tagged as Nano, non of them are a Macro...) And same for a "micro" character, that could be looming over a whole galaxy as long as someone else looms over them in turn. (Here the "micro" character obviously isn't a micro.) The same is the case for normal sized characters in a scene. They too get tagged as macro or micro all the time, despite clearly belonging to neither category.
The problem with the current tagging practices for macro and micro is that it leads to a lot of micro content technically being macro content, and vice versa.
It is like tagging small_penis in an image between two obviously hyper characters just because one of the characters is 40% bigger than the other, people searching for small_penis is obviously not searching for an image with only hyper penises in it. (and tagging small_penis on that image is considered tagging abuse if one does it often enough.)
Likewise a person searching for macro isn't after an image of a normal sized character hugging a tiny character in an environment with clear indications that there is no abnormally large characters in sight. And vice versa for someone searching for micro, they don't look for a macro looming over a building while getting loomed over by a bigger macro themselves. (or a macro looming over normal sized individuals)
The micro/macro tag is also a bit ineptly named and should likely just be Excessive_size_difference
In short, the micro tag should like only be used if the character is smaller than normal. While macro should only be tagged for characters that are larger than normal. And if there is no indicators to what is normal sized, then excessive_size_difference is still a sufficient tag to describe the content. There is technically no need to tag such an image with macro nor micro.
I did however also put "small" and "giant" into the title of this thread.
As already stated, "giant" has been aliased into "macro", making the macro tag imply anything from abnormally large but still realistic, and up to infinity and beyond. (Personally I think giant works best when said character is still within the realm of "realism"/lore and artistic freedom. Ie, loosely bound by physical limits. While macro has a complete disregard of the consequences of the square cube law.)
And I do see reason for having a "small" tag as well, for generally dwarf sized characters that are still technically realistically sized to a degree.
Some might ask, "But what about in between cases?" Like a image of a character that is a bit large to be a giant, but a bit small to be a "proper" macro. Here we can just tag both, since it is still an objectively large character. And the same applies for the boarder between small and micro.
The reason for having small and giant is to give room for abnormally sized but still loosely realistic characters, instead of shoe horning them into the micro and macro tags where they frankly do not belong.
So to end this post, here is my opinion on how the tags should logically be used:
Nano: The current Wiki description works fine.
Micro: A character that is more or less unrealistically small.
Small: A character that is noticeably bellow normal stature.
[Normal sized characters here]
Giant: A character that is noticeably above normal stature.
Macro: A character that is unrealistically large.
Giga: The current Wiki description works fine. (I however think "city dwarfing" is a bit small on the macro scale for Giga to start here, especially if we get the Giant tag back. Continent dwarfing is likely a more suitable start.)
Size_difference: When there is a noticeable size difference between the characters.
Extreme_size_difference: When the size difference is frankly comical or beyond. (Personally think beyond 10x in size difference is a good "lower end" for such a tag.)
If there is nothing to compare the characters to as to figure out what is "normal sized", then refrain from using nano, micro, small, giant, macro or giga. (Here one can make an exception if there is multiple characters on one side of the scale, like one tiny individual surrounded by multiple much larger ones, then "micro" is likely applicable, and vice versa for macro. (since the overall theme of the image is more in line with the normal content of the respective tags.))
There is also the question about if species should be accounted for or not, and to what degree, etc.
Now I hope people actually read this and share their own thoughts and opinions.
Personally I do see it as an improvement, however one that will require a lot of images to be corrected.