This column shows the correct length of each ROM image. In general, Kickstart ROMs prior to Kickstart 2.0 are 256KB (262144 bytes) in length, and those after are 512KB (524288 bytes). Exceptions to this rule are the Amiga 1000 Bootstrap ROMs (consisting of two 32KB EPROMs, or a 64KB ROM image) and the special Amiga 3000 bonus code appended to the end of A3000 SuperKickstart ROM images. Within the 64KB Amiga 1000 bootstrap, only the first 8KB or so (7816 bytes, to be specific) is actually used, although a full dump of the entire 64KB space is useful for those who may need to burn replacement EPROMs.
All CRCs calculated with GetCRC were done with ROM images of the correct size - if you have a ROM image with a matching Kickstart checksum but differing CRCs, this is one possible cause.
The most common "wrong" lengths of Kickstart ROM images are either a result of overdumps (where the image dumped contains the contents of memory space beyond the actual Kickstart ROM, such as the "mirror" of 256KB ROM images immediately after the "real" 256KB image) or Amiga 3000 SuperKickstart ROMs with bonus code attached (these ROM images should be either 279552 bytes in length for a 256KB ROM image, or 526572 bytes for a 512KB ROM image). Early versions of the transrom utility included with the UAE Amiga emulator mistakenly overdumped 256KB ROMs as 512KB images, containing two identical copies of the Kickstart image concatenated together. Stripping off the first 262144 bytes of such overdumped images will result in the correct Kickstart ROM image, although the emulator itself will accept the overdumped 512KB image without problems.
NOTE: There may in fact be multiple versions (of different lengths) of the Amiga 3000 256KB bonus code. If you have any details on this, please let me know.