The silicon photomultiplier (Si-PM) is a promising photodetector for a high-resolution PET scanner due to its small size, high gain and lower sensitivity to magnetic fields. There are several commercially available Si-PM arrays with different pixel sizes and fill factors, and these parameters can affect the performance of a PET block detector read out by these devices. We compared the performance of block detectors using 4 × 4 Si-PM arrays with 25 νm (Hamamatsu S11064-025P) and 50 νm (S11064-050P) pixels combined with the same 15 × 15 matrix LGSO block made of 0.7 × 0.7 × 6 mm3 scintillator pixels. Evaluated characteristics include photopeak linearity, energy resolution and positioning performance. Although the photopeak linearity and energy resolution are slightly better for the Si-PM with 25 νm pixels, the position performance measured by the separation of the position histogram is significantly better for the Si-PM with 50 νm pixels. We conclude that using the Si-PM with 50 νm pixels will provide a better solution for the development of ultrahigh-resolution PET systems.