The TV menu might label it a multitude of ways. My Panasonic plasma calls the mode that is considered bit accurate "Just". My Samsung doesn't really offer much in the way of settings, but it does allow custom sizing, so it's possible to zoom both width and height to the exact place you want. So you might have to experiment with different settings before you find the one that makes the Roku image fill the screen exactly.
Dan Roku Stick (3600), Ultra (4640), Ultra (4670), Premiere (3920), Insignia 720p Roku TV, Sharp 4K Roku TV, Nvidia Shield, Windows 10 Pro x64 running Serviio and Plex on a wired Gigabit network.