Most of those I report are "trivial" as defined by Corbin, in two ways: 1) expanded from shorter forms that are also transposals, mainly by pluralising (Adding-s to gerund nouns is legitimised by Webster's Third New International Dictionary [Web3]: see "[-ing.sup.3]".); 2) having more than a short stretch in common. Several are even ostrich anagrams, identical but for burying the first or last letter or so inside, and other metatheses. But all are unrelated in core words.
The longest clearly DEFINITIVE anagrams (Kidd's "synonagrams") are two or three pairs of exact and reciprocal synonyms. The first and third, even in singular, are not in any of the cited sources.
12-letters: parentalisms paternalisms--direct synonyms according to Web3 and Wikipedia
12: sterilizings Listerizings, or sterilisings Listerisings (ostrich anagram), or
14: sterilizations Listerizations, or-s-for-z-, Listerizations being a probable nonce
Two looser synonagrams showed up. The first and longest contains common suffixes that outweigh its core words, and streamlessness is a probable nonce.
16: masterlessnesses streamlessnesses--lackings of direction or 'channel'
14: reorientatings reintegrations--bringings into a new order or alignment
Four additional, arguable synonagrams are flawed by not being grammatically parallel words.
12: contrariness constrainers--opposers or counters
12: inoculations inosculation--two sorts of joining or intermixing (ostrich)
14: intercalations centralisation--insertions into the centre of something
12: memorialists immortalises--the latter is what the former endeavour(s) to do
Nine ANTI GRAMS are all but one ostriches or other poorly mixed metatheses, the 18 being the longest anagram I found or created. But in their favour, all are grammatically parallel.
13: altitudinally latitudinally--of opposing spatial coordinates
15: altitudinarians latitudinarians--high minded vs. broad minded, religiously liberal
12: altitudinous latitudinous--ditto
18: altitudinousnesses latitudinousnesses--ditto (nonces)
15: attentivenesses tentativenesses--fully vs. temporarily or uncertainly "there"
12: deifications edifications--deities cannot be improved
13: forwardnesses frowardnesses--of opposite personalities, progressive vs. difficult, contrary, perverse, retrogressive
12: peripatetics precipitates--on the move vs. settlings
14: ruthlessnesses hurtlessnesses
Six more possible antigrams are all grammatically unparallel words.
12: desecrations considerates (Considerates as a nonce noun, while not listed in dictionaries, is a common way of referring to individuals, by a trait, like "haves" and "have nots".)
12: despoliation depositional--reducing vs. increasing a bank's holdings
12: determinants detrainments--leave the 'trains' driven by determinants (Both are plural nouns yet unparallel types of words, an abstraction vs. real event.)
14: interosculates sansculotterie--mixes/integrates vs. divisive radical French Republicanism
12: rationalizes realizations (or-s-for-z-)--makeshift, biased defensive constructs vs. real understandings or direct awarenesses
14: derationalizes derealizations (or-s-for-z-)--undoings or losses of the preceding pair
Three weak DOUBLES (antonyms and 'synonyms'), an ostrich, an unparallel and a flip metathesis:
14: creativenesses reactivenesses--each opposes the other, vs. both create
14: doctrinairisms discriminators--being discriminating opposes indiscriminately adhering to doctrine, vs. the doctrinaire discriminate against 'non-believers'
14: rectifications certifications--antonyms: change your behaviour or get 'certified' (sent away); synonyms: officially certifying a correction, or the changes required to obtain certification
Four gramatically equivalent COGNATES of over 12 letters I leave to you to justify (or not), but I define uncommon words.
13: containerised inconsiderate
paternalities parentalities (paternal/parental states, personalities, conduct or policies)
14: nephologically phenologically (re clouds; re seasonal climate cycles)
15: disorientatings disintegrations
Seven more cognates are not parallel word types.
13: interoceptors retrospection (internal bodily receptors)
14: interrogatives tergiversation (contradictory, evasive or equivocal answers)
15: continuednesses tendenciousness (conservatively promoting a bias or partisan view)
deprotestantise predetestations (Inquisition or Jihad?)
16: counterdoctrines reconstructioned
ANIL, Perth, Australia