If anyone still trying to figure out the buffer size for EX4200. It is described as follows.
24 ports on EX4200= 1PFE (Packet-Forwarding-Engine)
My EX4200-has 48 ports + 4 uplink ports = 3PFE
Every 1PFE = has 3MB of buffer size.
Hence, EX4200 with 48ports + 4 uplink ports = 3PFE = 3 x 3MB = 9MB total buffer space available for shared buffer.
(Note: Found above info based on 2010 Junos CoS document, couldn't find any latest. Please also check attached gif, has explaination also for EX2200, EX3200, EX8200 or go http://www.juniper.net/us/en/local/pdf/implementation-guides/8010073-en.pdf )
Hope this Helps. Cheers!
- AJ