The abootimg.exe recognized it, so it is one those with that ANDROID! header (+ lzma'd kernel and gzipped ram disk(s)).
You can use those perl unpack-repack scripts floating around for this. Here is one version that hopefully works for it:
HOWTO: Unpack, Edit, and Re-Pack Boot Images (alternative method's zip file)
So if the Karbonn image type is the same (with one ram disk - or what else there may be), its size is then:
pagesize + padded kernelsize + padded ramdisksize
(kernelsize & ramdisksize are multiple of pagesize so they have "extra" zeroes at their end)