./mktorrent -a ANNOUNCE_URL -p -l 23 /LOCATION/TO/FILES/DIRECtORY

Piece Sizes

-l 18 = 256kb
-l 19 = 512kb
-l 20 = 1MB
-l 21 = 2MB
-l 22 = 4MB
-l 23 = 8MB
-l 24 = 16MB (some clients don't like 16mb pieces)

http://mktorrent.sourceforge.net/