You can use Movie Maker, an application embedded in your Windows operating system. If you don't have this software you can download and install it from the official web page.
Open the application and go to Import Video, select your MP4 file and click Open. Drag and drop the video to the Movie Maker timeline at the bottom of the window and click the Save to my computer button. Enter a name for your file, choose a place to save the video and click Next. Check the Best fit to file size box and enter the desired size for your video. Press again Next and Finish.
Also in order to reduce the size of your video, you can use other applications like: iSkysoft Video Converter Ultimate, iMoviesoft Total Video Converter, Pro or Advanced Video Compressor. All of them are easy to use and have a lot of useful features.