Q: How much fluid is needed at least to produce clinical symptoms in Pleural effusion? 

 
Answer: About 300 ml

Usually patient can tolerate mild pleural effusion without having any clinical symptoms.

Read "DIAGNOSIS OF PLEURAL EFFUSION: A SYSTEMATIC APPROACH" - AMERICAN JOURNAL OF CRITICAL CARE, March 2011, Volume 20, No. 2 HERE

Link: http://www.aacn.org/wd/cetests/media/a112002.pdf