Can you confirm the Resolution you have set for your Layout?
You can confirm the resolution you have set by selecting the Layouts option in your CMS, clicking the down facing arrow at the end of the Layout and choosing Edit from the menu. Next select the Background tab to see the Resolution setting.
If you have not created one already, I would recommend creating a Custom Resolution of 800x480. You can then set your Layout to this resolution so that the video fills the entire Layout.
Below is a guide for using LED Boards with Xibo. I would recommend following the instructions in the How to Create a Custom Resolution section.
Many Thanks.