Motivated by a recent magnetization reversal experiment on a TbFeCo thin film, we study a topological excitation in the anisotropic nonlinear sigma model together with the Zeeman and magnetic dipole-dipole interactions. Dipole-dipole interactions turn a ferromagnet into a frustrated spin system, which allows a nontrivial spin texture such as a giant skyrmion. We derive an analytic formula for the skyrmion radius. The radius is controllable by the external magnetic field. It is intriguing that a skyrmion may have already been observed as a magnetic domain. A salient feature is that a single skyrmion can be created or destroyed experimentally. An analysis is made also on skyrmions in chiral magnets.