invoke a .net service in designer

I had created a .net service which have in parameter a string in xml format.

I call the .net service in java service.
Uusually that’s work but if my string is too big (but not so big : for instance about 25 ko) I have a response in java which is : com.wm.dotnet.pkg.WmDotNetException: Reasons:Received a plain text response: , DetailErrorId:, DetailMessage

Is it a maximum length to call a .net service from designer ?

PS if I use web service provider to implements .net service and a web service consumer to consume the provider, I have the same error.