It's trivial to add another variable between the user interface and Makefile rules to turn the friendly value into what's convenient for the code.
Yeah. But then again, we already agreed that this is a very rare-use expert option. I don't think it's worth adding yet more lines to the Kconfig file just to make it pretty to choose.
The actually supported granularity is anything that CBFS/cbfstool supports as an image size, which I think is in theory arbitrary down to individual bytes.
What are the common CBFS_SIZE values?
We're using 1MB for most (all?) of our boards, but that's really completely arbitrary. If you want to use this at all, you probably know exactly what kind of size you want it to be as well. I really don't see the point in expending effort to restrict this to some arbitrary bin size.
(I'd also like to add that we're already developing a half-finished feature in our tree right now that will reshuffle the whole image layout once again, making this option obsolete... so we might not want to spend too much effort polishing it.)