How can I display only the most popular questions in full screen mode?

You can display on the projection mode the messages that gave rise to the most reactions. 

Discover how to do so in this article: How to use the reactions?

Note: reactions can be positive or negative, therefore top messages will be the most reacted in general. If you want to identify the most popular ones, you need to disable reactions other than the Like (thumb up) before your participants react to the messages sent.