Download Irfanview. (Do an internet search for it.)
It is freeware.
In the drop down menus in the software there is the option to resize the image so the longest side is no more than 1500 pixels.
You can vary your file size by lowering the quality of the saved file - about 75% usually does the trick.
I use it for posting photos that have a higher resolution. It resizes very effectively.