iphone s6 how to reduce picture size
How do you reset the camera to take a smaller size photo
iPhone 6s Plus, iOS 10.3.2
You cannot reduce the picture size other than by zooming in. Zoomed pictures will have a smaller picture size. Or use the front camera instead of the back camera. The FaceTime camera has a smaller picture size - only 5Megapixel instead of 12 Megapixel.
Or use iCloud Photo Library with the option "Optimize storage". Then your photos will be stored in iCloud, and Photos will remove the high resolution originals and replace them by smaller thumbnails for browsing.