angle ACB = angle HJB                                      (angle i = angle r)

angle ABC = angle HJC = 90

triangle ABC is similar to triangle HJC

JH/AB = CJ/CB = 1/2

2JH = AB.

also JH + AB = 4h

JH = (4/3)h.

if you are taking the wall as wide as the man, he wouldnt be able to see the wall behind him so length of mirror required is (4/3)h.

but if width of wall > width of man, length of mirror reqiured = (4/3)h + 2h = (10/3)h .