If you're using an Aero theme in Windows (which I think Windows 10 is all Aero themes), you might not be able to see the line which needs to grabbed with the mouse. Move your mouse around under the dialog, and watch for the mouse pointer to change to the double arrow icon. When it changes, then you can click-drag it.
Also, in Windows, if you have more than one dialog open, you can't resize a dialog. So it will only work if you have only that dialog open.
Note that keeping the Objects dialog open all the time can cause Inkscape to work more slowly. So it's a good idea not to keep it open constantly.