I'd also like the interface to be friendlier. What granularity is actually supported for the option? I doubt that it's single bytes?
Maybe make it decimal kb? The default value depends on ROM_SIZE, right?
I would very much prefer it to stay byte sized. This makes the Makefiles that use it easier to write and avoids confusion compared to the byte sized CONFIG_ROM_SIZE. (IMHO building a unit like kilo or mega into a variable or interface is always the less desirable choice because it adds another opportunity to screw up and become confused for the programmer. Sometimes it can be justified with the risk of overflow of the respective data type, but for Kconfig that doesn't count.)
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.