If you have the Roku Express+ model with RCA output, menus will be stretched vertically instead of letterboxed horizontally if you change the aspect ratio via Settings > Display type to 4:3 standard. Their content will not, 4:3 content will be filled, 16:9 content will be letterboxed horizontally.
You should be able to get a streaming player that letterboxes both the menus and 16:9 content, but fills 4:3 content in the 4:3 signal.
4:3 Letterbox and 16:9 Wide are names of aspect ratio settings for digital tuner converter boxes, DVD players and Blu-ray players.
I’m into videotapes and old electronics stuff, including extenders, converters and analog 4:3 TV sets (which are obsolete these days), and Miss Bianca is my favorite character in Disney’s The Rescuers (1977) and its sequel, The Rescuers Down Under (1990). I am not a Roku employee, only a user like you. ~ Jordan
Roku Express+ 3910X