Words for big, large & great in Celtic languages.

Words marked with a * are reconstructions.

Proto-Celtic	*māros = big, great *māyūs = bigger, greater *mārāti = to enlarge, magnify
Gaulish	maros
Lepontic	𐌌𐌀𐌓𐌖𐌉 (marui)
Old Irish (Goídelc)	mór = big, great mó, moü, moä = bigger
Middle Irish (Gaoidhealg)	mór, már = big, great mó = bigger, greater mórán, moran = a large quantity or number anmor = huge, enormous
Irish (Gaeilge)	mór [mˠoːɾˠ / mˠɔːɾˠ] = big, great, large mó [mˠoː/mˠuː] = bigger, greater, larger mórán = much, many anmhór = huge, enormous, very friendly anmhórán = huge amount, hugh number athair mór = grandfather baile mór = large town, city
Scottish Gaelic (Gàidhlig)	mòr [moːr] = big, great, large, grand, strapping; ample, bulky; high, lofty, tall; spacious; large amount mò = bigger, greater, larger mòran = a lot, many, much, multitude ana-mhòr = huge, innense, enormous, prominent baile-mòr = town, city mòr-chuid = majority, most
Manx (Gaelg)	mooar [muːr / muːɹ̝ / muːə̯ / muː] = big, great, grand, heavy, tall, chief, major, familiar, powerful, marked, commodious, intimate, capacious, extravagant, intense, extensive, grievous, bold (promintary), loose-fitting, difficult moo = bigger, larger mooaran = many, much mooarane = great deal, lot, many, much, multitude mooar-earroo, mooar-eash = majority
Proto-Brythonic	*mọr [mɔːr] = great, large
Old Welsh	maur = great
Middle Welsh (Kymraec)	máúr, maur, mawr = large, big moi, muy, mui, mwy = larger, bigger, greater muyhaw, mvihaw, mvyhaf, mwyhaf = biggest, largest, greatest mawraidd, mawredd = great, fine, grand, majestic mawr eir, mawrair, mawreir = boast, bragging, eloquence, elevated language mawrdec, mawrdeg = great and fair, very fine, magnificent, splendid mawrder, mowrder = largeness, bigness, immensity, greatness
Welsh (Cymraeg)	mawr [mau̯r / mou̯r] = large, big; fully grown; capital (letter); heavy (rain); long (hair/time); deep (water), great, greater, stormy, rough mwy [muːɨ̯/mʊi̯] = larger, bigger, greater, louder, more, longer, further mwyaf = biggest, largest, greatest, most, loudest, longest mawraidd = great, fine, grand, majestic mawrair = boast, bragging, eloquence, elevated language mawrdeg = great and fair, very fine, magnificent, splendid mawrder = largeness, bigness, immensity, greatness
Old Cornish	maur = big
Middle Cornish (CerneweC)	maur, meur, mûr = great, large, big, much moy = more, greater, bigger moya, moycha, mocha, mochya = greatest, most
Cornish (Kernewek)	meur [mø:r / me:r ] = great, grand, large, substantial, much moy = another, extra, more moyha = maximum, most meur lowr = considerably meur ras = thank you meuredh = majesty meurgara = to admire meurgarer = admirer meurgeryans = admiration meurgeryek = admirable meurgerys = beloved
Old Breton	mor = big
Middle Breton (Brezonec)	meur = big, very, many muy, mui = more meurbet = very, a lot, big meurded, meurdet = size, magnitude, greatness meurdez = majesty meurniver = multitude meurvor = ocean
Breton (Brezhoneg)	meur [møʁ] = big, many mui = more moyha = maximum, most meurded = magnificance meurdez = majesty meurvor = ocean

Etymology: from Proto-Indo-European *moh₁ros (great) [source].

Words from the same Proto-Celtic root, via Byzantine Greek μάραον (máraon – sweet chestnut), include marrone (brown, chestnut) in Italian, marron (chestnut, brown) in French, Morone (sweet chestnut) in German [source].

Proto-Celtic	*brassos = great, violent
Old Irish (Goídelc)	bras = boastful, strident, violent
Middle Irish (Gaoidhealg)	bras, brass, brassa = boastful, defiant, forceful, violent
Irish (Gaeilge)	bras = great, strong, swift (literary) brasach = lively, quick-spoken brasaire = lively, quick-spoken, talkative person
Scottish Gaelic (Gàidhlig)	bras [bras] = swift, precipitous, rapid, hasty, impetuous, impulsive, rash, quick-tempered, exuberant, heady bras-astarach =fleet-footed bras-mhacnas = exuberant mirth, extreme debauchery bras-uisgeach = swift/white-watered bras-shruth = rapids, torrent
Welsh (Cymraeg)	bras [braːs] = thick, fat, plump, stout, bulky, fatted, large, strong; coarse (sand); heavy (rain)
Middle Cornish (Cernewec)	bras, brâs = great, gross, big, large, coarse brasder = greatness, largeness, bigness, pride braslavar = grandiloquent brasoberys = magnificent brassa = greater
Cornish (Kernewek)	bras [bra:z] = big, bulky, large braslavar = boast, threat brasoberys = magnificent brassa = bigger, major braster = bulk, size brastereth = majesty brastir = continent
Middle Breton (Brezonec)	bras, braz = big, large, deep, important, strong brassaat, braçzaat, braçzeët, brasat = to grow, increase, put on weight, swell, extend, enlarge brasadur = extension, enlargement brasentez, brazentez = size, pride
Breton (Brezhoneg)	bras [bʁaz] = big, huge, important brazentez = size, magnitude, greatness

Etymology: from Proto-Indo-European *gʷrod-to- from *gʷred- from *gʰer- (to rub, stroke, grind, remove) [source].

Words from the same PIE root include gros (big, thick, fat, coarse, rough) in French, gross in English, and grosso (big, large, fat, thick, heavy, rough) in Italian [source].

Words marked with a * are reconstructions.

Sources: Wiktionary, Am Faclair Beag, Online Manx Dictionary, Teanglann.ie, eDIL – Electronic Dictionary of the Irish Language, In Dúil Bélrai English – Old Irish glossary, Geiriadur Prifysgol Cymru, Gerlyver Kernewek, Gerlyvyr Cernewec, Lexicon Cornu-britannicum: A Dictionary of the Ancient Celtic Language of Cornwall, Dictionaire Favereau, TermOfis, Le dictionnaire diachronique du breton, Geriafurch, English – ProtoCeltic WordList (PDF), Etymological Dictionary Of Proto Celtic