When cutting a hollow door in height - such as most of the lightweight white primed type of doors - it is best to make your decision on whether the door would look best with the door being adjusted from the bottom only, or the top and the bottom.
If you need to reduce the door by as little as 15-20mm this should be possible by trimming the top and the bottom without removing the small framework which gives the door its internal strength. Anything between 20mm and 50mm should be removed from the bottom.
Anything more than the above adjustment may be best reduced from the top and bottom... double the work, but a better result.
One thing to remember is that you may be able to re-use the tiny timber frame which will be in the piece your are cutting off, this should be possible if you are removing more than say 20mm, after cutting this amount of the door it is best to strip away the original door surfaces from the small timber frame (which is in the pice you have cut off) with a chisel, you can then trim the timber frame with a small plane.
Next, take the timber frame and cover the two surfaces that will be sandwiched between the existing door faces. Tap the frame gently into position, being careful to knock back the cardboard honeycomb internal area first. Try and place the door flat so that you can add wieght on the surface above where the framing has been refitted. Leave in this position for at least an hour, this way you will not need to use any nail fixings.
Cutting these types of doors in width is something I would not reccommend any layperson to try, there are real issues to trying to cut these doors without lots of frame clamps which are used in a proper Joinery workshop. Leave this to the experts.
The www.DirectDoors.com web site does have a search facility for some really oddly sized doors; you can search for doors at 457mm, 533mm and 610mm x 1981mm high, and also search for 526mm, 626mm, 726mm, 826mm and 926mm x 2040mm high. These last five sizes are all 40mm thick.
If your door varies a few mm from the sizes above I would personally not worry, they may well be the nearest thing you will find on the market. All other standard sizes sizes are normally available in all door types.
Any questions, email firstname.lastname@example.org
Info supplied by www.directdoors.com