Mammoth Uncensored Word List:
26 words ending with s, 24 letters long

cholecystenterorrhaphies dichlorodifluoromethanes electroencephalographers electroencephalographies friggatriskaidekaphobics hepatoportoenterostomies hydrochlorofluorocarbons hysterotrachelorrhaphies intercomprehensibilities lymphangioleiomyomatosis microspectrophotometries overintellectualizations paraskavedekatriaphobics paraskevidekatriaphobics psychoneuroimmunologists quadragintacentillionths quattuortrigintillionths quinquagintacentilliards quinquanonagintillionths quinquaoctogintillionths quinquaquadragintillions quinquaseptuagintillions quinquasexagintillionths quinquatrigintilliardths septuagintacentillionths undecillionsedecilliards