Also, it's 49152.

System variables were irrelevant after loading if you never called into ROM, but yes, that 6912 byte display block was fundamental. However some games and demos did steal a kb or two by using only a part of the screen and masking the rest with black on black color attributes.

In addition the lower 16KB was much slower than the upper 32KB since it was shared with ULA. So one had to make sure to place the code that needs speed in the proper part of the memory.