Many newer commercial dvds can't be ripped by DVDShrink anymore. Too many new copy protection schemes have been added in the past few years. I suggest ripping with RipIt4Me, which is a free program that combines DVDDecrypter, DVDShrink and FixVts behind one user interface. It makes use of Shrinks compression abilities if you want to use it.
If you are going from a commercial dvd though, the amount of compression you'd have to use to get down to 900mb would be way too much using Shrink. You'd get a terrible result. I suggest going to xvid or divx if you are going to save them to your computer.
Once you have them ripped to the hard drive, you can reencode them to divx in a program called DrDivx, or you can use AutoGordianKnot. Go look through some of the guides for dvd to divx over at www.videohelp.com