In reply to Mr. Lee:
A good mountain boot fit in terms of length should provide between one and one and half finger thicknesses of length beyond the length of the foot.
The way you measure this is by loosening the laces, moving the foot forward until the toe just touch the front of the boot and slip your hand down the back of the heel. When the tips of your fingers have reached the back of your heel then there should be plenty room for them and they should be able to move back and fore about half a finger thickness.
This isn't a millimetre accurate method but it is a practical tool-free method that scales to your body.
The room needed for easy and pain-free walking downhill isn't a lot different from what is needed for front-pointing but you might look for something nearer the one and half fingers when choosing a winter boot.
If you have that amount of length then in a boot that USED TO FIT there are a few other things to think about. Stretched boot and collapsed lining providing less support is certainly a consideration. Feet changing shape is another rarely-considered consideration. Your feet respond to their use level the same as other parts of your body and they can get fatter or thinner with the rest of you.
What about socks? Not the trusty OLD sock issue? Watch out for that one!
Insoles and volume adjusters change the shape of the boot volume in one way and different socks change the volume in a different way. Many people use two socks and typically you could change the thickness of the outer sock. Then there are changes in lacing. You may have to spend some time messing about with this stuff. Try to be methodical.