SOLVED. THANK YOU CREATIVE MEDIA TEAM and support unit for addressing my problem asap.
I called support unit and they have me connected to creative media team and they access my laptop screen and checked the problem and figured out the solution.
SOLUTION: I needed the serious system update. I didn't update my system for a long time and my imovie needed an update.
step 1: uninstall your imovie i.e, trash it
step 2: empty your trash
step 3: click on apple icon at the left top of the screen and go to system preference . Go to software update and upgrade your system
step 4: it will restart automatically
step 5: open it and go to apple store and install imovie
step 6: try exporting file of any size, it should work fine.
Mine worked perfectly fine. So satisfied. Thank you.
