Apple's OS X does not include software to easily extract a movie file from a commercial DVD. They do not want to be accused of fostering piracy.
DVD files are not standard computer movie files, so you have to extract a movie file from the DVD. Use Handbrake, free, for this. Compression is part of the file conversion. Handbrake will show the estimated file size when you choose the format and codec. A good choice for modern format is MP4, and a good codec for quality to size results is H.264. If you want to share the movie file with Windows XP guys, you should choose the old-style AVI format. To play AVI in QuickTime Player, install the codec included with the Perian package, free, link below.
If you need subtitles with your movie file, you have two choices: Use the option in Handbrake of embedding the subtitle by language choice into the extracted file. Use a downloaded separate .SRT file (allsubs.com) to allow QuickTime Player to show subtitles with an AVI or MKV file, or to allow VLC Player to show subtitles with an MP4 file.
Some useful terms:
-- burn = burn, for real. A laser is in the CD or DVD burner to etch the reflecting surface of the plastic disc material.
-- rip = extract a file from a disc (CD or DVD).
-- .CDR = uncompressed disk image (here disk includes disc); same as .ISO file. In OS X, Disk Utility can make a .CDR file from a group of files. It can also mount a .CDR or .ISO file as a disk. Changing the name from .CDR to .ISO makes no difference in mounting the image in OS X.
-- DVD format = disc with at least a folder called "VIDEO_TS" that has a group of files to make all the chapters and subtitles of a DVD movie.