I have a 3 display windows 8 system. Two are 24" monitors and one is a 27" monitor. The 27" monitor is in the middle and one of the 24" monitors is flipped portrait on the right. When I go to the screen resolution dialog box in Windows the 24" monitor and the 27" monitor appear to be the same size. This means that the edges of the window don't line up. Similarly the 27" monitor and the portrait monitor don't line up in the sense that when the mouse goes from the top of the 27" monitor to the portrait monitor, the pointer jumps downward. However if I go from the bottom of the 27" monitor to the portrait monitor the pointer lines up correctly and transitions smoothly.
The screen Resolution window correctly identifies my 27" monitor as a Dell S2740L and the driver is up to date. Also if it is relevant, the 27" monitor is hooked up using HDMI, the left monitor is hooked up using DVI and the portrait monitor is hooked up with HDMI -> DisplayPort with an Active HDMI to DisplayPort adapter.
Best Answer
The images of the monitors are relative to the pixel resolution. This will be why they all appear the same size regardless or physical dimension of the monitor.
Monitor Resolution