All those settings are not very consistent, and we should probably implement something easier to understand, but there is a maxMessageSize setting :
@startuml
skinparam maxMessageSize 50
participant a
participant b
a -> b :this is a very long message on several words
a -> b :0123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789
a -> b :return 0123456789012345678901234567890123456789012345678\n901234567890123456789012345678901234567890123456789
@enduml