I think what you mean is the small sidebar. In the original game you could not go over 640x400 as the resolution. When KeeperFX increased the resolution, at first the sidebar failed to scale up with the increased resolution and became extremely tiny on very high resolutions. However, many people do prefer the smaller sidebar that mid-sized resolutions provided. This is what you see in that video.
At some point the scaling of the sidebar was also implemented, and now whatever resolution you play at, the sidebar is always the same size. You can't change that, unless you go back to an old version of the game. You can still hide and show the sidebar by pressing the Tab key.
Changing the resolution is also still done by pressing Alt+R while ingame (not paused, not in a menu, but playing an actual level). And unless you have a 4K monitor or something, I suggest configuring it to your native resolution.
Edit: Looking at the video again, I see that person is playing in 'cluedo mode'. I think it's ugly, but some people prefer it. It can be enabled through the settings menu ingame. If you want to use that, it's best to keep it at 640x480 or 640x400 resolution, because that mode has never been implemented properly for high resolutions. You'll get clipping issues.