xD But genuinely, yeah a few things I use in addition to that bit about Babs vs Gideon and Babs vs the Tridentarii:
Colum has a whole fucking head on Ianthe (possibly give or take, this is said while they're mid combat so it could be a rough thing)
Pro is the tallest at Canaan House by a noticeable margin
Harrow struggles to touch the bottom of the pool, slipping fully under at first. Joli struggles to believe Nona is more than 14 at most, Hot Sauce (who is 14) has an easier time passing for an adult than Nona, and Harrow's compared to Jeannemary multiple times. When they dress Nona as Harrow they have to borrow clothes from "the smallest Blood of Eden member they could find" and then cinch those tight to keep them from falling off.
Muir in the tumblr post describes the twins as taller than even Palamedes, who Gideon describes as tall, though Corona has half a head on Babs "and a full head if you counted her hair" while having half a head counting her hair on Palamedes, which to me implies "half" is rough and she's only a tiny bit taller.
Ianthe has well over a head on Harrow.
Among some other bits. So absolutely these are all loose ideas for scale and you could scale them up or down, make some closer or farther apart, etc; this isn't definitive by any means. But some personal headcanons based on that info and on "hehe numbers are fun" for the fine-tuning are:
Babs - 5'8" (roughly average amab height)
Gideon - 5'9" (inch taller than Babs; hehe 9)
Harrow - 4'9" (TINY but not quite SO tiny you'd expect it to be way more emphasized; hehe 9)
Twins - 6'1" (plus ~3-4 inches of volume for Corona's hair)
Palamedes - 6'0" (so 4-5 inches "shorter" than Corona counting her hair; hehe 6)
Colum - 6'8" (7 inches taller than Ianthe; an average 'head' is 8-9 inches so still very close; hehe 8)
Pro - 7'0" (hehe 7; I also think 7'7" and built like that would be hilarious but probably pushing believability atp. xD Also Gideon describes him as "6 feet of sinew" at one point so I choose to believe that's roughly his height to the shoulder, not counting neck and head)
The other #forscale character we have is Augustine, who's canonically 6'0" even and the only character we've directly met with a specific canon height. The fact that he's very tall is the second thing Harrow notices about him so that gives us an idea that averages haven't changed much if any over time in this universe. So just to rapidfire some more thoughts, John is noticeably shorter than Augustine even while kneeling; G1deon is described as very imposing but only via his bulk, never described as super tall; Mercy is short enough to press her face into John's chest at the end seemingly easy enough. So for the rest of the Mithraem, I like to imagine:
John - 5'8½" (still within Average; 5/8/1/2 for him and his three closest people hehe; also half an inch shorter than Gideon as a treat)
G1deon - 5'10" (barely above/upper end of average, shorter than Augustine
Mercy - 5'3" (which is dead average for afabs yet frequently perceived as "short" and I feel like it'd be fitting for her to be so eternally fucking exhausted by this)
Bonus points: Ianthe being the tallest person on the Mithraem, if only barely. Mercy having dealt with people "looking down on her" (literally) for so long and then Harrow's tiny ass comes along and rather than defend her she's like finally lmao.
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