The Roku app use HLS or Adaptive Streaming, which is a protocol that detects your bandwidth and CPU capacity in real time, and adjusts the quality of a video stream accordingly.
This means that Roku will automatically chose the best video resolution, depending on your internet connection.
If your video looks blurry, is slow or has very low resolution, please reset your internet connection, get the Roku device device closer to your wireless modem, and try again.