18 letters

argilloareenaceous argilloferruginous argininephosphoric aristocraticalness aristodemocratical arsenophenylglycin arterioloscleroses arteriolosclerosis

19 letters

archconfraternities architecuralization aristorepublicanism

20 letters

archaeopterygiformes arteriosympathectomy arthroophthalmopathy arytenoepiglottidean

22 letters

arterionephrosclerosis arthropneumoradiogrphy

23 letters

arabinofuranosyladenine arthropneumoradiography

24 letters

argininosuccinicaciduria arteriolonephrosclerosis

Words with a ∙ are valid Scrabble words (SOWPODS word list)