Welcome to Android Central! If you downsize them, you'll lose resolution and clarity. If they're your only copies, that means you'll lose the original high quality images. How about backing them up to Google Photos, after which you can delete them locally from the tablet (since you can always access them in the Google Photos cloud, and re-download them if you want)?
https://support.google.com/photos/an...DAndroid&hl=en
If you really want to resize them, try a 3rd party app like
Photo & Picture Resizer.