Working with Layers - Adobe Illustrator CS6 on Demand - page 398

Set Layers Panel Options
Select the Layers panel.
Click the Options menu, and then click Panel Options .
Deselect the Show Layers Only check box to show all layers. Select it to show
only top-level layers and sublayers.
Click an option for Row Size: Small , Medium , Large , or Other , and then enter a
size (12-100 pixels).
Select the check boxes you want for Thumbnails: Layers , Top Level Only ,
Groups , and Objects .
Click OK .