Door sizes are actually frame opening sizes- the doors are always a bit smaller so the can be opened and closed. Unless it is a custom, something like 29 3/4" is rounded up to 30" which is a nominal 2'-6" door and frame size. 80" is the usual height, but some older homes had slightly shorter doors- some new doors can be ordered to a 78" height as shipped. I just buy off the shelf and cut them down. With today's hollow core doors if you cut more than 3/4" off the bottom you'll have to reset the botton peice. It may need the facing cleaned off, use a sharp chisel and don't worry about small gouges. Carpenter's glue and some spring clamps will reset it. While the glue is drying start on the next door.

Condoman has the right technique so long as the old door fit correctly which with settling it may not have. With extreme care all you need is a skilsaw with a very sharp blade, a sharp chisel, and the drill and bits to bore for the lockset (usually a 2 1/8" holesaw for the knob and either 7/8" or 1" for the plunger body). Sandpaper backed with scrap will smooth the cuts you make and don't forget to round over the cut edges a little so the door facing will not catch if it's brushed up against. Maintain at least 1/2" in the clear on the bottom for HVAC return air on interior doors. Doors ar simply an exercise in planar geometry so any 6th grader should have an understanding of how to do them right but you'd be surprised at how many messed up door installations I find. Think of the big picture first, then cut or adjust and all will be well.