Few lines from the Best practices guide regarding the Maximum Distance between members in Virtual chassis..
Interconnecting Members in a Virtual Chassis Configuration
A Virtual Chassis configuration consists of between two and 10 EX 4200 series switches connected together through Virtual Chassis ports. Configurations can range from a deployment in which each switch is physically collocated, to a deployment in which switches are separated by up to 70 km and connected via fiber optics. Each switch within a Virtual Chassis configuration is a member of that Virtual Chassis configuration.
http://www.juniper.net/solutions/literature/implementationguides/801018.pdf