I am having the same issue with a Roku ultra on new Samsung tv with 4K HDR at 60 Hz HDCP 2.2. Happens watching NBC channel shows within the ROKU. Always when a commercial comes on and I have to go back to the ROKU home to get the display normal size and then return to the show., Last occurred binge watching Chicago Fire S8E5.
I am having the same issue on my Roku streaming Stick when I'm watching any show on the NBC app especially days of our lives what happens during a commercial break and then continues to shrink to a small window even during a show the only temporary solution is to hit the fast-forward button and then press play and the screen returns to normal. It's very annoying it's been going on for over a month Roku needs to fix this ASAP.
Same here on Roku TCL smart tv watching The Good Place. Commercial goes small, then the program resumes small as well. Using back arrow to exit and enter brings it back to full size. Also, the voice picture synch goes bad during commercials, need to pause ans resume to synch.