If you are using a Mac, Photos (older versions are named iPhoto) will do it. Just export a copy of the photo to a location and save it as a medium or large file size instead of the default full or original size. Preview will also do it by exporting with a different quality setting. If using Preview, its best to rename that export so the original file and size is not overwritten.
I don't know what Windows has in the way of built in photo programs anymore but maybe someone else can fill in that information. If nothing is built in, there is a free download called Irfanview
http://www.irfanview.com that will do the resizing as well as converting to other formats and a lot of other functions. An excellent program but it has been years since I used it. As I remember, you just opened the photo in the program and then resaved it under a different name and in a different size. The purpose of the different name is again to keep the original file and size intact.
There are probably several other Windows programs. Kev had posted information some years ago in the FAQ on one program. If it is still there it may work but I don't know if it works with the latest versions of Windows.
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