How to record photo/video files smaller on iOS

My iPhone photos and videos are too high resolution so that I can't send them via email or facebook messenger.

I rarely need high-definition photo and video, and I'd like to set the default resolution much lower than it is out of the box.

Is this possible? And is it possible to shrink the file size of my existing large photos and videos?

Try using Video Slimmer app in app store.. This app lets you to shrink video files filmed on your iOS device to smaller file size while keeping good video quality. With just several taps, you will greatly save the precious space on iPhone iPad or iPod.