When I load an HDR image into an environment texture using Cycles Render, only a small fraction of the image appears. I have a feeling it's an easy fix, but I can not find what needs to be changed.
When HDRs are used as Environment texture they they completely surround the scene, using an equirectangular projection. Think of it like big sphere around the scene.
It is natural then you would only see a portion of the image, which corresponds with the angle of view of the camera.
The main use for this kind setup is to use the HDRi image to light the scene in a very realistic way. In a way every pixel of the image becomes an emitter that contributes to the overall illumination.
If that is not what you are after, but want to use the whole HDR image as background image that fills only the frame, then set the image as Image Texture instead of Environment. Note that this will prevent the HDR image from effectively lighting the scene, and the perspective of the camera might not align correctly with that of the background image.