3 letters:
c's, c.s, cep, cia, sap, seo, sep, sip, siu, sop, sup, syp

4 letters:
beep, c.'s, cece, cede, cees, cele, ciel, deep, diep, heap, jeep, keep, leap, leep, lepe, peep, reap, reep, saez, sapp, seal, seam, seaq, seas, seat, seed, seek, seel, seem, seen, seer, sees, seib, seif, seis, seow, sepp, sieg, siek, siem, sies, sikh, sipe, sipp, soap, sope, sopp, soup, tepe, veep, weep, seep, seip, sepe

5 letters:
cease, cheap, heape, kneip, kniep, riepe, scene, sea's, seale, sease, seeps, seese, seife, seige, seith, seize, serpe, sheep, siebe, sieck, siege, siess, sleep, steep, sweep

6 letters:
seeped, seethe