If this is a TV with separate Roku attached...
Also, check your TV itself for a setting that adjusts picture size (could be labeled "zoom", "size", "picture", or something similar). On most modern sets, this can be controlled separately on each input.
Roku Community Streaming Expert
I am not a Roku employee, only a user like you. Please, no support questions via private message -- post them publicly to the Community where others may benefit as well.
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Ultra 4800 | Streaming Stick 4K+ 3821 | TCL Roku TV 43S245/C107X