How can I change the file size of photos uploaded to Google Photos?
Go to photos.google.com. Select Options from the top left of this page, then select Settings near the bottom of the Options menu.  Change the radio button to High Quality (free unlimited storage).  Just under this you will see two options - Buy Storage - Recover Storage.  Select Recover Storage and you will be presented with the option to "Compress".  Select Compress and it will warn it could take up to an hour to complete.  Go ahead and you will have then compressed all your original sized photos to High Quality and recovere the space.  If you don't have the full size photo's backed up, be aware this option is not reversible.