1 Big App: Thumbnail Creator – creates proportional thumbnails.
All you have to do is pick a preset size or enter your own.

Select images/photos from “Photos” or from “Files”.
Select a preset size
Switch ON to enter custom width & height.

Select images/photos from “Photos” or from “Files”.
Pick a preset size or  enter custom width & height.
Resize and Save to “Photos” or share anywhere 🙂