4 letters:
berg, burg

5 letters:
baggy, bergh, bergs, berny, berty, beury, birdy, birky, bogey, bogie, buggy, burby, burge, burgo, burgy, burly, burry

6 letters:
baggie, berg's, berger, bergey, berkey, berney, bernie, bertie, biggie, birdie, birkey, birley, birney, birnie, boogie, burger, burges, burgio, burkey, burley, burney, burpee, bursey, fergie, virgie

7 letters:
burgher

8 letters:
burghley