Assuming that you just have Paint, open up the image in paint, hit ctrl+w, and in the "stretch" part of the window enter 50 and 50 in the boxes labeled horizontal and vertical. Then save the image again. It should be 50% smaller.
If it's still too big just keep doing that.
Mom to Anabelle and Lila, rescued ladies and Harley, Corgi/Beagle/Basset/?? mutt
"Saving just one dog won't change the world, but it surely will change the world for that one dog." - Richard C. Call