Try typing WindowsLogo+Down (hold down the Windows Logo key and hit the down-arrow key).
If that doesn't work, try resetting LR's preferences: Resetting the lightroom preferences file - updated - Lightroom Forums. (The preferences file stores the last window location for LR's various windows, and deleting the file may cause LR to use the default size for the Locate window.)
This problem is not an Adobe (LightRoom) problem, but a very disturbing bug in Windows 10. The File Open/Save dialog box can be made bigger, but cannot be shrunk. This problem happens in a lot of applications such as many Adobe Applications, Corel PaintshopPro, Autodesk 3ds Max etc.
In Windows 7/8.1 shrinking the File Open/Save dialog box works normal.
A workaround for this problem is to delete all the keys in the windows registry under HKEY_CURRENT_USER/SOFTWARE/Microsoft/Windows/CurrentVersion/Explorer/ComDlg32/CIDSizeMRU.
This will reset all the all the positions en sizes of the File Open dialog boxes off all the software used under the given userprofile
I'm sorry tohave taken a while to get back you you, been busy...
Leov, I'll post response to Microsoft using the feedback within Windows 10 to see if that helps.
This problem wil be fixed in Windows 10586 (TH2) which will be released around november 10th. 2015