Reshade is a quick and easy online resizer. You can choose to use files from your computer or from the web. Without an account you can upload up to 3 images a day, and you can't crop them. It's quick, easy & had not downsides that I could find.
I also found PhotoZoomPro 4, a program that seems to do the same thing, but at $219 is more than I want to spend for just an occasional need.
I also read about SmillaEnlarger. The author's website was down when I tried to look into it, but I read so many things about it online that I decided to try it and found a free download here http://smillaenlarger.en.softonic.com/. (Just be careful to read all the messages when you download it here, they try to get you to add a bunch of extra stuff that probably makes money for somebody but I didn't want cluttering up my desktop.) So far it's been amazing. You just drag an image from your computer & decide how big you want it to be based on a percentage zoom factor, specified height or width, or other options to stretch or fit within boundaries. I haven't used it much, but what I've done as been surprisingly easy and straightforward for a free program. I highly recommend it!
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