The Roku allows you to set its output to match your display type, i.e. HD is 16x9 widescreen, SD 4x3, and SD 16x9 widescreen. A standard definition program will appropriately keep its aspect ratio when viewed on a widescreen monitor and appear with black "pillar" bars on either side. You can't force a picture that was filmed in one aspect ration to be shown in another unless you use your television's picture mode to stretch or distort the picture.
You can use the Roku player to adjust the screen size, but it really depends on the TV show or movie your watching. Most TV shows and movies automatically come up in full screen, but occasionally the TV show or the movie will come in on the screen on a smaller size, then the actual size of the TV screen. It happens rarely so it is not a problem for me.