I have used Irfanview for many years; it's a great little package for basic image viewing and manipulation - and it's free with no nasty ads or spyware attached. I always set it as the default viewer for jpg files on my computers and any I setup for friends and family.
Download it here: https://www.irfanview.com/
Irfanview offers many ways to change the size and resolution of a file (Image -> Resize/Resample image), but for reducing a file to a set size simply open a file, click on File->Save as, tick set file size on the small JPEG/GIF save options box and enter the desired file size in kB.
Simples.
Steve.