A 32 bit computer can address 2^32 memory so . the maximum process size will be 4GB.
The size of the virtual memory is the maximum size of the maximum size your computer can address using pointers. so a 32 bit os will have a 32 bit address bus. so by 32 bit address bus we can address 2^32 bytes.
while 64 bit computer or os means it uses 64 bit address bus. so it can address 2^64 bytes of memory.