3 letters:
ast

4 letters:
bast, cast, fast, gast, hast, kast, last, mast, nast, past, rast, vast

5 letters:
asked, blast, brast, caste, chast, clast, erast, fasst, ghast, glast, icast, jfast, kaste, laste, mast-, naste, pcast, plast, raste, snast, vfast, vlast

6 letters:
-assed, -blast, -plast, aghast, amcast, blast-, bypast, damast, dbfast, dicast, divast, dynast, e'last, g.fast, gassed, locast, massed, nefast, palast, passed, recast, remast, ryfast, sassed, strast, thrast, uncast, upcast, wtfast

7 letters:
agelast, amassed, anycast, arblast, attaste, at last, bedfast, belfast, biassed, brassed, bumbast, by-past, classed, comcast, diecast, dismast, fantast, forcast, glassed, go past, grassed, icecast, lambast, mercast, miscast, netcast, nonpast, nowcast, offcast, ooblast, orgiast, outcast, outlast, parcast, podcast, precast, rogfast, sitfast, sunfast, topmast, unicast, vinfast, vodcast, webcast

8 letters:
altacast, antepast, apoplast, backcast, bioblast, bioplast, blogcast, buckfast, bypassed, chiliast, coldcast, contrast, cragfast, deskfast, die-cast, downcast, elegiast, endocast, epiblast, everlast, fly-cast, fly-past, forecast, foremast, forepast, forncast, gamecast, gymnaste, handfast, harassed, headfast, holdfast, jurymast, lambaste, lockfast, mainmast, makefast, medifast, mimecast, miracast, molecast, myoblast, newscast, opencast, outcaste, overcast, overmast, overpast, phantast, pleonast, polycast, portlast, qualcast, repassed, smodcast, statcast, stedfast, symplast, telecast, the last, tooncast, top-mast, trispast, typecast, umbecast, wirecast, wormcast

9 letters:
acid-fast, acroblast, aftercast, alloplast, antispast, audiocast, autoplast, backblast, bandobast, beechmast, broadcast, cablecast, coloplast, colorcast, colorfast, crevassed, cybercast, cytoblast, cytoplast, declassed, dreamcast, duroplast, ecdysiast, ectoblast, encomiast, endoblast, endoplast, entoblast, etioplast, fail-fast, gigablast, gonoblast, half-mast, halfassed, hazelcast, hemoblast, hesychast, hold fast, holoblast, homoplast, hydrocast, hypoblast, idioblast, intercast, jury mast, leaf cast, lightfast, lipoblast, meroblast, mesoblast, mesoplast, metaplast, microvast, mitoplast, mizenmast, monoblast, monoplast, multicast, neuromast, open-cast, outgassed, parablast, periblast, periplast, pleonaste, polyblast, polyspast, pyroclast, quantcast, roughcast, royalmast, sand-cast, sandblast, sand cast, shamefast, shoe-last, shoutcast, simulcast, smashcast, soothfast, standfast, starrcast, steadfast, superfast, surpassed, teloblast, tonoplast, transfast, type-cast, ultrafast, undercast, very fast, windblast, xenoblast

10 letters:
adenoblast, ameloblast, aminoplast, amyloplast, angioblast, apremilast, archiblast, astroblast, back-blast, bare-assed, bomb blast, breastfast, chiroplast, chromecast, cnidoblast, colloblast, colourcast, colourfast, cuban bast, cystoplast, dermoblast, ecclesiast, elaioplast, enthusiast, fibroblast, forepassed, gymnasiast, gymnoplast, gypsoplast, half-assed, hard-assed, iconoclast, idoloclast, jiggermast, laserblast, leucoblast, leucoplast, leukoblast, leukoplast, light-fast, lithoclast, march-past, metaphrast, microblast, mizzenmast, multifaced, myeloblast, mythoclast, narrowcast, neuroblast, neurospast, osteoblast, osteoclast, osteoplast, outclassed, overpassed, paraphrast, pendergast, pentaspast, phenoplast, planoblast, protoplast, rockpalast, royal mast, rubriblast, sarcoblast, sarcoplast, screencast, silverfast, solo blast, sporoblast, sportscast, stand fast, statoblast, symposiast, thronecast, trade-last, trespassed, triplecast, voltaplast

11 letters:
biblioclast, brachyblast, centroblast, chromoblast, chromoplast, countercast, cranioclast, disdiaclast, elastoplast, embryoblast, flabbergast, foretopmast, gonimoblast, hematoblast, immunoblast, kinetoplast, lymphoblast, megaloblast, melanoblast, needle cast, nematoblast, odontoblast, odontoclast, odontoplast, orbitoclast, plasmablast, prendergast, rebroadcast, sideroblast, smart-assed, spermoblast, spheroplast, spongoblast, tear-gassed, thomas nast, tight-assed, tillinghast, trophoblast, trophoplast, unsteadfast, unsurpassed, weathercast, whirl-blast

12 letters:
at long last, cementoblast, chirogymnast, chondroblast, counterblast, erythroblast, fore-topmast, granuloblast, haematoblast, haematoplast, lecithoblast, mooring mast, nesidioblast, next-to-last, phragmoplast, plaster cast, spongioblast, thromboblast

13 letters:
adamantoblast, blepharoplast, entosthoblast, hard-and-fast, hemangioblast, hemocytoblast, myofibroblast, pansporoblast, pronormoblast, spermatoblast

14 letters:
cobbler's last, first and last, haemangioblast, leukocytoblast, phagocytoblast, slartibartfast

15 letters:
cytotrophoblast, radio broadcast

16 letters:
weather forecast

17 letters:
religious outcast

18 letters:
kaliningrad oblast

19 letters:
syncytiotrophoblast

21 letters:
dead hand of the past

6 letters:
arafat

8 letters:
pyrostat

10 letters:
bureaucrat