Open the Gallery app
Select the photo you want to resize
At the bottom of the screen, click the edit button - shaped like a pencil
In the top right, click the more options (3 dots) and click Resize image
Select the size for the image to be resized to then click "Done" to submit the changes.
Click "Save" to save the image with the new size - this is irreversible.
If you want to also keep the original full size image, click the 3 dots in the top right and click "Save Copy"(Note that this only an option for newer devices or devices running more recent versions of Android.
If you want to keep a copy of the original full resolution image using an older version, we recommend saving the image elsewhere before making these changes.