My game window is very small and does not resize automatically. What can I do about it?
Go to Options->Graphics and disable "Don't stretch/shrink the map".
Also keep in mind, the client follows the scaling of Windows to fix problems with scaling on high resolution displays. You can either set the scaling of Windows to another value or "Disable display scaling on high DPI settings" in the compatibility options for the client.exe in your Tibia folder.