I found chunk size:
Chunk Size of 4096
Jun 22, 2011 – Chunk size is 4096 bytes in system.img. Spare size calculated to be 128 bytes.
Creating working folder WORKING_062211_155053 ...
Copying boot.img and system.img ...
Adding an update-script ...(This is the example in the webpage not sure if the person who posted this actually ran it)
Chunk size is 4096 bytes in system.img
Spare size calculated to be 128 bytes
Setting chunk size = 4096 in unyaffs.c, line 20
Setting spare size = 128 in unyaffs.c, line 21
Compiling unyaffs ...
unyaffs successfully compiled
Extracting contents of system.img ...
Hope that helps