The image is expected to be an environment map, and for best results you should use an HDRI; you can find many good free ones here: https://hdrihaven.com. Various sizes are offered for each, it is your choice how much resolution you need.
It is also possible to map flat across the output image plane setting Projection: Camera (Fit) or Camera (Stretch) in the image dome settings in the GUI. Just try starting IPR in the GUI and you can quickly see what each option does.
The output image size, as exported from the plugin, will be determined by the SketchUp viewport size. However, in the Bella GUI, you can select the camera and set a different resolution if you wish. Without a license, regardless the output resolution you set, it will be restricted to 720p by area (1280×720), and will be reduced keeping the aspect ratio of the resolution you have set.