On iPhone: In the Photos app, tap the image to open it. Tap the “info” icon beneath the image, or just swipe up on the image itself.
On Android: Go to Photos, select your image and click the "..." (three dots) icon in the top right. Scroll to bottom of page to find image size.
The image details will be displayed, including some or all of the following: date, time, and image filename camera and lens information, image size in megapixels (MP), width/height pixel dimensions, and megabytes (MB) exposure settings (if available) location information (if available) Width/Height
Pixel Dimensions are the most helpful attribute for estimating the best print size for the image. Please keep in mind that image size is different than image sharpness. Photos that are not in focus will loos worse (not better) when enlarged.