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Sorry, it's not possible. You can use \n to break message.
Or you can use maxMessageSize parameter. For example:
@startuml skinparam maxMessageSize 100 A->B:very long long long message name @enduml
http://www.plantuml.com/plantuml/uml/SoWkIImgAStDuIhEpimhI2nAp5N8JQpmJIqkJanF3SwiIbKmD33WSjIrSx8gIosgLCZ9pqj78d8X2XNo4dDJkHnIyr90HW40