4 bytes (32 bits) or 8 bytes (64 bits)

Computer Systems A Programmer's Perspective Second Edition

Buses
Running throughout the system is a collection of electrical conduits called buses
that carry bytes of information back and forth between the components. Buses
are typically designed to transfer fixed-sized chunks of bytes known as words. The
number of bytes in a word (the word size) is a fundamental system parameter that
varies across systems. Most machines today have word sizes of either 4 bytes (32
bits) or 8 bytes (64 bits). For the sake of our discussion here, we will assume a word
size of 4 bytes, and we will assume that buses transfer only one word at a time.