The best option is to inflate the ball to its correct size (rather than inflating it to a specific PSI). This means that if you’ve purchased a 26-inch ball, you should use the pump that comes with the ball and continue inflating until the ball measures 26 inches in diameter.
There are two things to keep in mind here:
Don’t over-inflate the ball, as this will increase the chance of it bursting.
Some brands of yoga balls need to stretch a bit when they’re brand new, so if you’ve just opened the box to start inflating your ball, go easy and inflate to 80%, leave the ball overnight, and inflate the remaining 20% the next day.