Keeping select2 element a fixed size with scrolling?

http://jsfiddle.net/8svf11yh/1/

.select2-container .select2-selection {
    height: 60px;
    overflow: scroll;
} 

Or overflow:auto; may be a better choice

A few refinements to @smcd's version:

.select2-selection--multiple { max-height: 10rem; overflow: auto }

I am using max-height so that it can be smaller if only a few options are selected. Only once it reaches that height does it grow no bigger. As suggested overflow: auto is great as it only uses the scrollbar if needed. Finally I target the --multiple variant of the selector so only selections with multiple options get this styling.