Problems with understanding memory map STM32F446RE
The actual size of the bootloader is completely meaningless. You should never have to worry about it, and I doubt that STM will ever commit to guarantee any definitive size. It may well be different for each chip revision. The fact that the memory just above the bootloader is declared "reserved" indicates that STM reserves the right and space to change/extend the bootloader in the future. Again, you shouldn't be concerned with that; just treat the bootloader like every other section of "reserved" memory: Don't touch it, and don't rely on anything being readable from it.