There are three SIM sizes. Doubt it would take a regular SIM, but either a Micro or Nano. Can you check?
BTW - AT&T will give you a free SIM and add this to your plan for $5/mo. or $15/mo. stand alone.
That being said, I found I almost NEVER use my watches as stand alone phones even though AT&T has free NumberSync. So save your money. It’s a FAD.