6 letters:
mastek, mastic, nastic, tastic

7 letters:
-nastic, blastic, clastic, drastic, elastic, mastick, plastic, plastik, spastic

8 letters:
-blastic, -plastic, aclastic, aplastic, chiastic, dicastic, doxastic, dynastic, entastic, ergastic, gelastic, jelastic, monastic, orgastic, pepastic, silastic

9 letters:
animastic, bombastic, chalastic, dichastic, emplastic, epinastic, epipastic, euplastic, fantastic, fantastik, gymnastic, gymnastik, inelastic, lantastic, onomastic, orgiastic, peirastic, poptastic, sarcastic, unelastic

10 letters:
anaclastic, anaplastic, bioclastic, bioplastic, bitumastic, chiliastic, docimastic, dokimastic, dysplastic, epiblastic, epispastic, hyponastic, myoblastic, neoblastic, neoplastic, nonelastic, pederastic, phantastic, pleonastic, proplastic, scholastic, stochastic, symplastic, synclastic, zooplastic

11 letters:
aeroelastic, alloplastic, antiblastic, anticlastic, antiplastic, antispastic, astochastic, autoclastic, autoplastic, cacoplastic, cataclastic, cataplastic, ceroplastic, cytoclastic, cytoplastic, ectoplastic, encomiastic, endoblastic, entoblastic, entoplastic, epicerastic, esemplastic, gum elastic, hesychastic, holoblastic, homoblastic, homoplastic, hudibrastic, hypoblastic, hypoplastic, idioblastic, meloplastic, meroblastic, mesoblastic, metaplastic, monoplastic, neophrastic, nyctinastic, paederastic, parablastic, paracmastic, paraplastic, phonetastic, polyplastic, pornerastic, predynastic, pyroclastic, scholiastic, unsarcastic, vasospastic, xyloplastic, zymoplastic

12 letters:
ameloblastic, amphiblastic, amyloplastic, angiospastic, antigalastic, antiorgastic, antiphrastic, antonomastic, cosmoplastic, cyberspastic, desmoplastic, deutoplastic, diploblastic, discoblastic, ecclesiastic, enthusiastic, fibroblastic, fibroplastic, gymnoblastic, haemoplastic, holophrastic, hyperplastic, iconoclastic, metaphrastic, myeloblastic, orthoclastic, osteoblastic, osteoclastic, osteoplastic, paraphrastic, paronomastic, periphrastic, protoplastic, rhinoplastic, superplastic, viscoelastic

13 letters:
amino plastic, chloroplastic, cryptoclastic, embryoplastic, heteroblastic, heteroplastic, homoscedastic, keratoplastic, lymphoblastic, megaloblastic, melanoblastic, nonneoplastic, odontoblastic, organoplastic, pancratiastic, phelloplastic, plagioclastic, thermoplastic, trophoblastic, visco-elastic

14 letters:
antineoplastic, dermatoplastic, erythroblastic, erythroclastic, fibrinoplastic, galvanoplastic, haematoplastic, neo-scholastic, paraneoplastic, stomatoplastic, thromboplastic, unenthusiastic, volcaniclastic

15 letters:
achondroplastic, autoschediastic, blepharoplastic, heteroskedastic, interscholastic, porphyroblastic

16 letters:
overenthusiastic, phenolic plastic

20 letters:
fluorocarbon plastic

24 letters:
trip the light fantastic

5 letters:
aspic, asdic