How can I insert a censor bar into a mp4 movie? I have a feature film in mp4 format. There is one scene where I just want to place a black censor box to conceal one part of the screen, no more than 7 seconds.

I'm a beginner, so my concern is that if I import the two-hour, 700MB file into, say, Windows Movie Maker, make the minor change, and then convert the finished product back into mp4, then the finished file will have a huge size that won't fit on a 4.7GB DVD.

Is that a valid concern? Are there video editing softwares out there that somewhat retain original file size?
Any decent video editing software will allow you to specify bitrate and resolution in export settings. Sony Vegas Pro or Movie Studio, Adobe Premiere Elements, as should Cyberlink Powerdirector among others.

For a 2 hour movie with 700MB as output size, you want a target or average bitrate of 670 kbps for the video and 128 kbps for the audio.