Try searching this forum or the '8.x' forum for 'compression' maybe you find some answers there.
Compression al depends on the type of data you want to compress. Word documents you can compress, avi or mpegs not. If you combine them there is now way of telling if it fits on a tape. Just try it out. If you regulary run out of space you might consider to use bigger tapes. (if your tapeunit can handle them, otherwise you wil needl a new tapeunit also)