Resizer 

The resizer (top center between panes) is like a splitter. You can drag the 4-headed arrow to adjust the relative size of each of the panes.

To resize the entire dialog itself, drag any of the edges or corners of the dialog, or expand it to fill the entire display by clicking on the maximize button in the top-right corner of the dialog.

The three panes in the object selector dialogs look and act somewhat differently than the panes of the main window. They are not separated by splitters; instead, the single 4-headed arrow is used to resize them as a group rather than a pair at a time.