To reduce the size of these cheap hollow doors ...
Keep the closing side as it is, as it has the lock block
Mark the required width with a line top to bottom to the hanging side
Prize the hardboard up off the timber frame and remove the timber
Clean off any glue residue so that the timber is smooth
Now you need to insert the timber at the line to the required width
Remove any of the cardboard filler which may obstruct the timber when it is inserted
Now, either glue the timber,, and insert to the line, and cramp it up (pin the harboard too if need be)
or
trim some of the excess hardboard off first, to make inserting the timber easier
The frame at the top and bottom of the door should then be screwed to the edge frame - but be careful if you need to reduce the top or bottom, so sink the screw heads as required
It only takes 15-30 minutes, and if you use rapid setting glue, you can hang the door straight after