Bus width is measured in bits. Each letter of the alphabet or character uses 8 bits or one byte of storage.
Bus size can vary from 8-bit, 16-bit, 32-bit or 64-bit. Simply put, an 8-bit bus can carry one character at a time, a 16-bit bus can carry two characters at a time, a 32-bit bus can carry 4 characters at a time, and a 64-bit bus can carry 8 characters at a time.