Is it the address bus size or the data bus size that determines "8-bit , 16-bit ,32-bit ,64-bit " systems?

Generally the size of the databus is determined by the the size of the processor registers. Often it is the size of the processor registers that determine the OS type (64 vs 32). The physical bus sizes can technically differ from this (8088 as an example) but it's so rare that the author of your quote probably associates the two together.

Generally the pointer size also follows the register size but the physical address bus width can be bigger (as with the 8086 16-bit at 20) or smaller (as with the AMD 64 at 48)