Hi, B. Hollow doors are made kind of like a face frame with plywood glued to both sides. They also have cardboard wrapped around on edge in the hole for a little extra support and vibration reduction--at least ours have. Since the door has solid wood on the 4 edges, you can trim the length slightly. Up to a half inch should be no problem. This assumes that your doors are still standard height and haven't already been trimmed down a lot.
Whatever kind of saw you use, have a nice sharp blade and use a straight edge to cut against if possible. The plywood door skins are splintery so an 80 tooth blade is best if you are using a circular saw. Some people put masking tape on the cut lines as they think that the tape reduces splintering. This has never worked all that well for me but you may wish to try that.
If you can work really accurately, there is another method. On one side of the door, you can make a cut with a utility knife or even make a i/16" deep rabbitt with a circular saw. Then make the full depth cut from the other side about 1/32" further down the door so the cut mark stops the splintering. All that said, the sharp, fine-toothed blade is still the biggest secret.