I know the question might sound a little wired but here is the issue I accidentally faced with my Windows 10 PC. You can follow the below instruction to replicate the same issues in your Windows 10 Environment.

If you right click on the task-bar and select and of the following options, i.e.
  • Cascade windows
  • Show windows stacked
  • Show windows side-by-side

Click image for larger version. 

Name:	2015-08-01_18-08-17.jpg 
Views:	120 
Size:	29.6 KB 
ID:	27418

you will see that all of your windows sized will be automatically changed to either very small or very large. Now I didn't find any way to undo it and bring my windows size to the previous default one (what was after the Win 10 installation) except manually streaking each window to almost the default size, but again it's not perfect.

So do any of you guys know a way to change the window size back to normal what we get after installing the Windows 10 OS? Please share if you found any solution.