Some gamers may see that on their laptops Sonic Mania is in a small window and they cannot resize or expand it. But it is possible to turn it fullscreen within the game settings. To do this, start up the game.
Then use your controller or keyboard to navigate to options. To play any platform Sonic game I would recommend a console controller.
Click on video once in options. Change Window Size to x2, and turn fullscreen on using the joystick on your controller or the arrows on your keyboard. Click on Y to apply the settings (confirm it and all that)
Now try playing Sonic Mania. Because of a bug, it may require you to confirm a few times but once you have confirmed it is recommended you close and open Sonic Mania again, but there you should have it. If you are encountering issues, comment down below. I should get back to you.
I hope it is working for you, and if it is not, remember to leave feedback!