Lot of users face the issue of  having only one by third screen size when they try to watch a video in Kodi. I am going to outline steps below by which you will be able to troubleshoot and fix this issue.

Steps:
PART A:
1: Start your box as normal.
2: From the home screen click on Settings and if you are using an amlogic device like T8-S you will see a colourful settings page. If you are not using an amlogic device you can still follow the steps that i will outline in PART B section of this article.
3: Now once we are on colourful settings page go ahead and click on Display.
4: Now Turn off the Auto Detect HDMI option.
5: Now you can manually set the resolution on the next option(HDMI Output mode setting), you can set to what suits best according to the capacity of your TV
6: Set display position this you can adjust (increase or decrease) according to your screen size.
7: Now once the above is done press the Home Button on remote and it will take you back to the Home Screen.

PART B:
8: Now we will clear data on Kodi by going into Settings again and then Other and then More Settings.
9: Now go onto Apps and then click on Kodi and then Force Stop(click OK on the popup) and Clear Data(click OK on the popup) .
10: Now press Home Button on remote and click on Kodi Or XBMC (whatever you are using).
11: Now go onto System option and click OK on remote.
12:  Now click on System option at the bottom, once you are inside system option at bottom left side you will see Settings Level, make sure settings level is on Expert. If it is on basic or standard just click on it and it will change to advanced, click again and it will change to Expert.
13: Now once the settings level is changed click on Video Output at the top.
14: Now go onto Video Calibration and click on it and adjust your screen according to your Tv Screen, make sure the corner lines are visible. You can also refer to the below video to understand the above steps easily.