Can I change the file size on a photo?

You can export pictures in different sizes. When you choose "File>Export>Export nn Photos" you get a dialog that allows you to choose a Size. It gives you some suggestions, or you can choose "Custom" to enter the size you want:

Did you really want to have a different sized picture within Photos? You could export the size you want to your desktop, say, and then import that image back into Photos. Then you would have two pictures of different sizes. But usually you just keep the original and export what you need.

Size in the export dialogue refers to the dimensions of the image length by breadth, measured in pixels. If you want to change the file size then the setting to use is Jpeg Quality - this varies the amount of compression user when creating the exported file. Maximum quality is least compression, hence largest file size. Lowest quality is the most compression, and the smallest file size.