Check the flash config, it will tell you what size NAND you have. Here is a list (taken from free60.org):
FlashConfig - System Types
01198010 - Xenon, Zephyr, Opus, Falcon
00023010 - Jasper 16mb
008A3020 - Jasper 256mb
00AA3020 - Jasper 512mb
Note that if you have a 256/512MB (Big Block) NAND, you only need to read 64MB (-r64 switch in NandPro).