Thumnail size

Hi could you please provide a function to change the thumnail size generated both when importing reviews via csv and when the user creates them on the front end? Our photos are not squared, but vertical. The resizing of the long side makes the short side really small and as the image container on the front end has 100% width, some photos look really pixelated. What we need is a snippet that instead of resizing to 500px the longest side of the photo, it does it with the shortest side. Meaning that for a photo 1000×2500 the photo is not resized to 200×500 but to 500×1250. Thank you