Word size is how much data the CPU can process at a time. The CPU manufacturer determines this when they design the CPU.

The older CPUs could only process 16 bits at a time. This number then doubled to 32 bits. CPU manufacturers are constantly pushing the limits to increase this number.

The important thing to remember is, the larger this number the more data your CPU can process at a time. In other words, the higher the number, the faster your PC.