I am interested in How to calculate the size of ROM and RAM.
Why, your code must know in order to function.
with out scanning the whole memory
The problem here is the balance between internal and external ROM or RAM, if you do not have the switches applicable to the derivative in place (i.e. you code knows) you may or may not have access.
It seems you are trying to make an OS that work regardless of the '51 derivative installed - that is impossible (different SFR addresses, different memory schemes, different built-in peripherals).

Erik

on another note: I seond the opinion "why an OS?"