Add "-Dmagicdraw.pageSetupDialog=true" property at JAVA_ARGS in mduml.properties* file, located in MagicDraw install root, "bin" directory. It will enable magicdraw page setup dialog in which media size could be customized manually by typing size in inches.
* If you are using MagicDraw 17.0.5 or later, the name of the property file is magicdraw.properties.