3 letters:
ass, cas, das, gas, ras, sas

4 letters:
alas, bass, bras, cass, lass, mass, nass, pass, sass, tass

5 letters:
amass, brass, class, crass, frass, glass, grass, kvass, masse, tasse, trass

6 letters:
admass, alsace, badass, bypass, camass, chasse, harass, morass, repass, strass, wrasse

7 letters:
air gas, donbass, impasse, jackass, surpass, verglas

8 letters:
air mass, bare-ass, blow gas, burgrass, coal gas, crevasse, eelgrass, en masse, eyeglass, ice mass, landmass, low mass, midgrass, nutgrass, outclass, overpass, ribgrass, ryegrass, seagrass, sea bass, spyglass, subclass, sunglass, tear gas, town gas, trespass, wild ass

9 letters:
age class, art class, bay grass, bentgrass, bluegrass, bur grass, candlemas, contrasts, cordgrass, crabgrass, cut glass, dog grass, forecasts, hall pass, high mass, hippocras, hourglass, ideal gas, inert gas, isinglass, jenny ass, knotgrass, low-class, low brass, make pass, marsh gas, martinmas, nerve gas, noble gas, nut grass, pin grass, plexiglas, red brass, rest mass, ricegrass, rock bass, rushgrass, rye grass, sandglass, sassafras, sewer gas, silkgrass, tallgrass, underpass, water gas, wineglass, yardgrass

10 letters:
barn grass, bear grass, beer glass, bell glass, bent grass, black bass, bromegrass, bunchgrass, cheatgrass, cord grass, crab grass, deer grass, fiberglass, fibreglass, form class, goat grass, groundmass, hair grass, hand glass, high-class, high brass, june grass, lead glass, lemongrass, life class, love grass, lyme grass, mardi gras, milk glass, opal glass, pier glass, plexiglass, poison gas, quackgrass, reed grass, rice grass, shortgrass, shot glass, silk grass, smut grass, soft glass, sour grass, star grass, stone bass, superclass, supergrass, tape grass, underclass, wheatgrass, wire glass, wire grass, witchgrass, wool grass, word class, yard grass

11 letters:
alpha brass, arrow grass, atomic mass, bahia grass, blowing gas, bottled gas, bottlegrass, broom grass, bunch grass, cabin class, china grass, cloud grass, couch grass, crown glass, curly grass, dallisgrass, devil grass, double-bass, double bass, field glass, first-class, first class, flint glass, flute glass, gallowglass, goose grass, grama grass, ground bass, lower-class, lower class, manna grass, meadowgrass, means grass, mustard gas, natural gas, naval brass, panic grass, penny grass, peppergrass, perfect gas, plate glass, plume grass, quack grass, quick grass, screen pass, string bass, sweet grass, sword grass, third class, upper-class, upper class, watch glass, water glass, witch grass, world-class, yellow bass

12 letters:
alkali grass, bahama grass, barley grass, bottle grass, brandy glass, bristlegrass, canary grass, carpet grass, cereal grass, channel bass, cotton grass, domestic ass, ecclesiastes, fescue grass, figured bass, finger grass, forward pass, ground glass, hardinggrass, junior class, lateral pass, laughing gas, manila grass, meadow grass, middle-class, middle class, object glass, orange grass, pampas grass, pepper grass, piece of ass, producer gas, quartz glass, rhodes grass, ribbon grass, ripple-grass, ruling class, safety glass, second-class, second class, senior class, silver grass, slough grass, social class, sparrowgrass, striped bass, switch grass, tumble grass, velvet grass, weatherglass

13 letters:
bearing brass, bermuda grass, biology class, boarding pass, bristle grass, buffalo grass, english class, foxtail grass, harding grass, inertial mass, levy en masse, liqueur glass, looking-glass, looking glass, mountain pass, natural glass, optical glass, orchard grass, pane of glass, physics class, poverty grass, prairie grass, ravenna grass, rock sea bass, soluble glass, stained glass, thorough bass, tourist class, whitlow grass, working-class, working class

14 letters:
barnyard grass, black sea bass, center of mass, drinking glass, egyptian grass, fountain grass, freshman class, sea lyme grass, sheet of glass, tall oat grass, venetian glass, volcanic glass, wild oat grass, windmill grass

15 letters:
admiralty brass, alligator grass, cartridge brass, chemistry class, coronary bypass, evergreen grass, freshwater bass, genus sassafras, hungarian grass, immigrant class, laminated glass, old witch grass, pain in the ass, salt reed grass, sophomore class

16 letters:
african wild ass, giant star grass, graduating class, laughing jackass, magnifying glass, major form class, water star grass

17 letters:
broom beard grass, korean lawn grass, reed canary grass, reed meadow grass, relativistic mass, tall meadow grass, velvet bent grass

18 letters:
african love grass, feather reed grass, gravitational mass, hairy finger grass, interplanetary gas, kentucky bluegrass, shatterproof glass, snake in the grass, spotted black bass, weeping love grass

19 letters:
american star grass, carbon monoxide gas, common cotton grass, continuing trespass, creeping soft grass, crested wheat grass, european dune grass, japanese lawn grass, kentucky black bass, korean velvet grass, sunday school class

20 letters:
conservation of mass, pheasant under glass, relative atomic mass, sergeant first class, yellow bristle grass

21 letters:
australian reed grass, japanese carpet grass

23 letters:
creeping windmill grass, liquefied petroleum gas, relative molecular mass

27 letters:
fairway crested wheat grass, law of conservation of mass

4 letters:
hats, katz, bats, cats, fats, rats

5 letters:
spats, flats, pat's

6 letters:
that's

10 letters:
and that's