Recommended image file size should be less than 2000KB.
Create an image that is 640px by 1138px or 1080px by 1920px.
On the iPhone 5s, iPhone 6(Plus), and iPhone 7(Plus) when a static image is opened in preview mode. This caused to cover a small portion of the top and bottom of the image. The user can see the full image when touches the image in preview mode and the top and bottom navigation bars are removed. We don't recommend to put any "important content" in the top 129 pixels, or the bottom 89 pixels of your MMS image.
We recommend a maximum of 2 MB file size of any attachment. In the case of Video, we have seen keeping videos around 30 secs would give you a file under 2000 KB.