Here's what I think it is...
You are exporting the Camera using the one from the specified Scene.
But you are not
setting a 'Resolution' or exporting that to Octane - therefore Octane chooses its own resolution for its rendered image, and when that doesn't show all of the Scene in the rendering-pane you don't 'see' what you expected.
Solution = put a 'check' in the 'Resolution' box - it's just above the Camera check-box - you can leave the height/width at 100% [this is the current size of the Sketchup window] or I suggest you adjust to smaller size [say 50%] to do these test renders.
Re-Export and Render and the Octane image should now be the same as the Sketchup Scene but at a somewhat different size.
Play around with the image size in the Resolution settings to suit your needs once you have it working...
Let me know how it goes.
The units you use don't matter - the Exporter converts everything into 'meters' anyway to suit Octane.