  We compute holographic three-point correlation functions or structure
constants of a zero-momentum dilaton operator and two (dyonic) giant magnon
string states with a finite-size length in the semiclassical approximation. We
show that the semiclassical structure constants match exactly with the
three-point functions between two su(2) magnon single trace operators with
finite size and the Lagrangian in the large 't Hooft coupling constant limit. A
special limit J>>sqrt(lambda) of our result is compared with the relevant
result based on the L\"uscher corrections.
