Increase size of Display Values field select in VBO Dialog UI for usability

Problem/Motivation

Data managers have a hard time selecting fields from the "Display values" select element in the Views admin UI. It's quite small in height by default and when you get lots of content types difficult to browse.

Proposed resolution

Let's add a #size to the display values form element in modify.action.inc. Adds a #size of 10 to the select element.