In reply to Katzina.W:
Too narrow boots you would know about; blisters and discomfort on the sides of the feet.
Too big or wide, and your feet slop and slide about, leading to blisters in various places.
Too small will cramp and damage the toes.
If the boots are the right length, but have too much volume, your feet can slide to the end when going downhill, resulting in toe and nail damage. This can be addressed by a volume-reducing footbed, and by experimenting with different lacing methods, to prevent the feet moving.