Most spa pumps are going to be offered with a port size of 1.5", 2", or 2.5". Some spa pumps will have a different intake and discharge port size. For example, some pumps will have a 2.5" intake and a 2" discharge.
If you are replacing an existing pump, it's important to go with a pump with the same port sizes. The problem is that it is not clear to many spa owners what they should be measuring to ensure they get the same port size.