We studied the size dependence of the ferroelectric domain structure in unique free-standing PbTiO3 thin film, composed of grains 60-1000nm in size, with transmission-electron microscopy. With such samples, we showed that the apparent dependence of electrical properties on the thickness of polycrystalline thin films stems from their grain-size dependence. We found that a domain-structure transition, from predominantly multidomain to predominantly single domain occurs at a grain size of; 150nm corresponding to a thickness 200-300nm. Based on these experimental results, the drastic change of coercive field and dielectric permittivity below a thickness 200-300nm was reasonably explained as resulting from a domain-structure transition.