Hey @Screetpeep, do you have that problem only with Tomb Raider or with all/more games on Stadia? If it's only this one game, it could be a problem of the game itself with your TV or TV resolution.
I would anyway recommend to check out with Stadia Support if they have any troubleshooting steps to help you or what you can do in this case. It might also worth checking out with the manufacturer of your TV if it might be a problem with the TV itself.
For myself: I have only tried out Mortal Kombat 11 and Samurai Shodown yet and both games worked automatically properly on the screen, so I didn't had to adjust anything.
I know you had this problem a while ago, but I wondered if you had found a solution?
I experienced this screen size issue when using Stadia on a non 4K TV at a friends house; the relevant setting was buried in the TV's menu.
TV - Settings - Advanced Settings - Picture Overscan - Off.
This will obviously change depending upon TV manufacturer. However, I thought I would share on the off-chance it will benefit someone else.