I expect that your new pistons will all be the same size. Just take them all to the machine shop and let them measure all four to be sure. Then they will fit the bore size to the pistons. No worries.
When you're into a job like this, your chance of success is greatly increased when you "worry" about these details. It's the person who doesn't give these things a second thought that ends up with lots of problems, so keep on "worrying". Attitude is everything.