The launcher config window presents errors when trying to set screen resolution. Run the game once to generate the config file, then navigate to the share/Steam/steamapps/common/Sonic Adventure 2/Config directory and edit UserConfig.cfg to edit screen resolution.

Fullscreen="1" for full screen or 0 for windowed

Width="800" Height="600" change numbers to match your screen size RefreshRate="60" change to match your screen refresh rate

After saving, set UserConfig.cfg to read only to prevent the launcher from changing these values each time you run the game.

The game runs too fast on 144hz monitors, download and extract d3d9 wrapper into the main Sonic Adventure 2 directory and edit d3d9.ini to limit the frame rate to 60hz. https://github.com/ThirteenAG/d3d9-wrapper