1 letter:
r

2 letters:
ar, or, er, r.

3 letters:
bur, mir, for, jar, npr, par, fur, mar, war, far, oar, ore, gar, ear, car, ayr, tor, sir, are, ire, cur, nor, bar, tar, per, fir

4 letters:
deer, lore, more, sour, slur, poor, lour, gore, torr, rear, door, bore, weir, year, tyre, amir, dire, boer, veer, were, spar, gear, hire, boar, lear, core, hour, star, boor, near, sore, tier, sure, jeer, yore, wore, pore, soar, moor, sear, meir, cure, four, doer, fear, fore, carr, scar, your, bier, parr, burr, fire, czar, wire, pour, dour, tsar, blur, birr, lyre, tore, hear, sere, mire, beer, char, spur, emir, dear, pure, ruhr, hoar, seer, lure, mere, thor, purr, peer, stir, here, tour, roar, barr, adar, pier, abor, fahr, gere, igor, jour, lohr, voir

5 letters:
score, loire, moore, assur, barre, whore, crore, churr, queer, snore, skier, ameer, sitar, drear, qatar, whirr, chore, scour, defer, cheer, afire, store, amour, spore, dower, senor, demur, steer, safar, corps, shirr, babar, unbar, we're, chirr, inure, shore, floor, myrrh, spear, sneer, inter, quire, shear, adore, dakar, abhor, incur, shire, sheer, dinar, bazar, flour, zaire, spoor, occur, refer, blear, starr, clear, moire, infer, deter, spire, smear, decor, murre, swore, recur, cigar, asarh, ashir, astir, athar, bihar, damar, debar, greer, grier, kotar, lamar, maher, sapir, selar, vanir, vidar, yasir

6 letters:
lekvar, appear, impure, secure, velour, insure, guitar, drawer, career, confer, sphere, ensure, assure, immure, bazaar, aspire, adjure, galore, vigour, endear, disbar, unsure, endure, ignore, squire, retire, concur, rehear, tumour, cohere, valour, severe, prewar, detour, mature, before, qintar, poseur, ashore, vapour, prefer, manure, you're, veneer, demure, revere, adhere, savour, voyeur, allure, admire, expire, abjure, anshar, bonior, cottar, devoir, exmoor, fakeer, inhere, isobar, jamjar, kaffir, lenore, lontar, menhir, nuphar, pindar, rigour, safire, salvor, sambar, sambur, schorr, secpar, shamir, shmear

7 letters:
austere, compere, verdure, myanmar, respire, rapport, pasteur, unclear, inshore, liqueur, premier, anymore, schmear, require, deplore, pioneer, inquire, postwar, saviour, masseur, cashier, inspire, sincere, restore, bizarre, couture, compeer, danseur, succour, obscure, fervour, procure, explore, flavour, implore, besmear, bonheur, co-star, cooccur, flexure, gravure, guinier, hachure, houhere, kildeer, languor, minibar, nervure, outroar, rancour, samovar, shophar, soilure, tangier, tarsier, tonsure, tussore, vitharr, wafture

8 letters:
brochure, grimoire, overhear, nightjar, polypore, reappear, engineer, reinsure, seigneur, souvenir, cavalier, offshore, saboteur, mutineer, frontier, frotteur, grandeur, immature, domineer, monsieur, reassure, premiere, perspire, transfer, abampere, abattoir, allosaur, amanpour, baccilar, barosaur, coiffeur, coiffure, coinsure, elaphure, encolure, ex-mayor, footgear, footsure, hot-wire, killdeer, lavalier, longueur, markhoor, microbar, muleteer, outscore, paramour, peignoir, scimitar, sinecure, tamanoir, twoscore, voussoir, wheatear

9 letters:
balladeer, guarantor, buccaneer, puppeteer, superstar, chevalier, raconteur, disappear, insincere, musketeer, persevere, premature, gondolier, volunteer, profiteer, interfere, bandolier, racketeer, sightseer, chauffeur, financier, brigadier, belvedere, apatosaur, archosaur, bandoleer, basketeer, cannoneer, cared-for, fourscore, gadgeteer, gazetteer, guinevere, hadrosaur, hekmatyar, hoped-for, kala-azar, megathere, nothosaur, pistoleer, presbyter, privateer, pterosaur, rest-cure, sloganeer, sonneteer, stegosaur, trochlear, warrantor, wasteweir

10 letters:
quadrature, bombardier, chandelier, charioteer, escritoire, commandeer, underscore, heretofore, auctioneer, ankylosaur, baudelaire, belshazzar, blackamoor, brontosaur, carabineer, ceratosaur, cockleburr, embouchure, gondoliere, halberdier, harpooneer, ictodosaur, kieselguhr, longed-for, looked-for, man-of-war, pelycosaur, plesiosaur, re-explore, scrutineer, seismosaur, smoothbore, threescore, tug-of-war, uintathere, wished-for

11 letters:
pamphleteer, uncared-for, litterateur, connoisseur, rectilinear, ichthyosaur, interlinear, candidature, corythosaur, dromaeosaur, dyaus-pitar, liquidambar, milliampere, rotogravure, styracosaur, supersedure, tyrannosaur, ulaanbaatar, unhoped-for, volt-ampere, yearned-for

12 letters:
entrepreneur, restaurateur, aide-memoire, heliogravure, photogravure, psittacosaur, staurikosaur, uncalled-for, unlooked-for, unwished-for

13 letters:
conventioneer, conservatoire, porte-cochere

15 letters:
kilovolt-ampere

16 letters:
pachycephalosaur