Paginate results of entity reference widget type
Found a nice article related to Entityreference Multiselectors here.

You can choose one from variety of options mentioned in above article which suggests using of


Using of Multiple Selects. 
Using of Chosen
An additional possibility is the Dynamic Multiselect Widget. 
Another option is Entityreference Views Widget
There is jQuery UI Multiselect
And Improved Multiple Select


Hope one of the above works for you :-)