For smaller size, you would need to transcode them into a format such as DivX. That can be a lengthy compression process, though it's possible to squeeze it down to a few hundred megabytes while maintaining decent quality.
If you have a pretty large HD, the simpler option would be to create images of the discs on your HD and mount them as a virtual drive. You can also just copy the contents of the DVD (AUDIO_TS and VIDEO_TS) into a directory and play it back with some player software.
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