The game resolution cannot be changed at will, it remains fixed at 3840x2160 if you have a 4K monitor. Or 1920x1080 if you have a FullHD monitor. There is no Fullscreen option, but there is only Virtual Fullscreen (Borderless fullscreen). It is possible to force fullscreen by editing the "user.pso2" file, but this is not something the average user likes to do. Many have complained about an incorrect resolution in Virtual Fullscreen mode, and that is solved only by editing that file.
All these problems are also present in the JP version. But it would be nice if at least here the devs will find a solution by inserting the internal resolution scaler option just like any other good videogame. For example, I have a 4K monitor, but I would like to play PSO2 in 1080p in virtual fullscreen.