@Olo My time to SHINE
1- Yes. Just to hammer in a point: there is no censorship on AO3. The only rules are that you must tag your work appropriately. If it has rape, tag it. If it has incest, tag it. If it has suicide, tag it. Just use common sense. And if you aren’t sure, there is always the catch all: “Author chose not to use Archive warnings.” That will always cover your butt. The “moderation” on AO3 is self-moderation; if you don’t want to see something, don’t search it, filter it out, don’t read it, etc. It’s an archive, not social media.
2- The answer to all of your questions is to simply create your own tags. You want to tag it as “M/f”? You type M/f and then a comma. Voila, it’s tagged M/f. The only reason you would find it in the list that pops up when you start typing is if someone else has opted to use that exact configuration before you. The tricky thing here is that because of the vast variety of prose AO3 caters to, having “canonical” tags in original works would be… basically impossible. The GOOD news is that you can use MULTIPLE tags in order to appear in as many search results as possible. So go ahead and tag it with all the terminology you can think of: “M/f, male giant, big man, giant man, tiny woman, shrunken woman, tiny female, size difference” so on and so forth. There is NO LIMIT to your tags. You would shit if you saw the literal wall of tags some stories have. I am not joking. So go crazy. Tag away.
For relationship tags: ditto. Just type it in how you like it and end it with a comma.