how do I resize my photo?

Photos for Mac is always keeping any photo we import in the original size and quality for loss less editing.

When you need a smaller, resized version for sharing, select the photo and use the command "File > Export > ..Export Photo". In the Export dialog you can specify the size and quality. To get to the details about the size and quality, click the down arrow "v" to the right of "Photo Kind: JPEG"

If you need a specific smaller size, set "Size" to "Custom and enter a new width for the exported photo.

You will find the smaller version in the folder you specify in the file chooser after you click export.