Hi!
You image need 2:1, example 8000x4000. You image i think example 8000x4400.
If you have done the rendering, you can fix it in the video - image editor, just change the image size to 2:1.
I recomended using Projection 360 (Use 6 cam) max limit quality 8192, in projection (360 Use 1 cam) max limit quality = 2048 px, if need more, need recompile source engine.