Hi Rootheday,
First, I used native resolution of both displays (1920*1080 for the main and 1680*1050 for the second). Also, I had checked second display with another laptop did not have any problem.
Second, I got your point about automatically scaling up in Windows 8.1. I think it is ridiculous because device manager is not third-part application; it is a Windows built-in application!! Actually, I scaled down it and then got a better image for device manager but other problems (mouse problem, other apps not fitting and so on) occurred that I inclined to ignore it. However, all of this story is about main screen and scaling up or down did not affect second screen.
Thanks for your reply.