Layer Properties > Symboloby > Show:

[JFBellemare]

Make the "Show" box (Features, Categories, Quantities, Charts, Multiple Attributes) wider. 
It's not like the contents change - ESRI knows what the options are, why not make the box the proper size?

While we're making boxes the right size, why not resize the Symbol-Value-Label-Count one too?
Especially the Value column, it needs to be much wider.

[RichardFairhurst]

The whole dialog should be resizable.  I have layers with over 300 symbol categories (sign faces) and you can only see about 6 symbols at a time.  I should be able to resize the horizontal size when I have many codes grouped together and the vertical size of the dialog to be able to see as many values as my screen will fit when I need it.

[GuyMadgwick]

I agree with rfairhur24's comment completely. My layer has over 70 symbols that also have very long descriptions and it is a pain to have to do so much horizontal and vertical scrolling just to see all the detail. It is also a pain changing the order of any of these symbols because the up and down buttons at the right hand side only allow one place to be moved up and down with each click so it can take ages to move a symbol all the way to the bottom or top of the list (a drag a drop option for this would be most convenient).

[KoryKramer]

Check out https://community.esri.com/ideas/4459‌.  The entire symbology dialog has been redesigned in ArcGIS Pro, answering the requirements of Richard and Guy above.  Also, Madgwick‌, in Pro you can now select multiple symbols and move them with the arrows.  There is drag and drop in the Symbology pane currently, and it looks like some improvements will likely be coming to that functionality as well in future releases of Pro.