Just having this problem when I choose full rect in the layout the scoreboard and everything with it is way too large. When i preview the scene i can only the a part of the HSeperator and thats it. The anchors are placed correctly though. Im using a 3440x1440 monitor, has it something to do with that maybe? Thanks in advance!
It most likely has to do with your display size, yes. It sounds like you're using an ultra-wide display.
Choosing the full rect layout option will make the interface automatically resize to fit the game window. If when you run the game the window expands to match your monitor's resolution, then that's why the interface is also resizing. You can select the node set to full rect and choose instead Layout -> Center to prevent it from changing size in the running game.