You would do your statistical sampling. Take 10 people and see how many will do the second click and you have your answer. My bet is half will click your event link and half of them will click to view full size.
About 2 to 3 out of 10 people is my bet.
I put an event on Facebook. I scanned a full-page flyer and uploaded it. The problem is that it doesn't fit neatly in the area above the description. I need to click the event then click the picture again to view the full-size ad. I am curious if most people will know to do this extra click.