I have a similar problem with Youtube - whereas yesterday I could see several thumbnails in at least 2 rows, now I can see literally 2 (and the tops of 3 on the next row).
I assume then that this isn't a YT app issue but one with the underlying UI features. I did think the player had decided it was 4k and thus showing double sized (even though my roku ultra doesn't do 4k, its set to 1080p) or that the app had broken, but if you're having the same issues then its probably Roku at fault.
looking into it a little more, it seems Youtube is slowly rolling out a new update UI that has much bigger thumbnails. The web and my TVapp also have the same new, big, unusable thumbnails so I doubt its Roku now. Does surprise me that your Roku app has this too - maybe its using Youtube under the covers as a rebranded YT app