Do you write WSDL manually or does the library create one for you looking at the endpoint class? If you encode WSDL manually, you can simply add something like this in the schema descriptor:
<simpleType name="MyStringType">
<restriction base="string">
<minLength value="10" />
<maxLength value="30" />
</restriction>
</simpleType>
<element name="greetMe">
<complexType>
<sequence>
<element name="requestType"
type="tns:MyStringType"/>
</sequence>
</complexType>
</element>
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