Thanks for your reply,
Adding the jbpm.execution.hbm.xml in my classes folder, changing the type to "text" and modifying the type of the column in the database to "clob" did the trick :-)
Hope it doesnt break anything else... B-)
<subclass name="org.jbpm.pvm.internal.type.variable.StringVariable" extends="org.jbpm.pvm.internal.type.Variable" discriminator-value="string">
<property name="string" column="STRING_VALUE_" type="text"/>
</subclass>
alter table jbpm4_variable rename column string_value_ to string_value_2;
alter table jbpm4_variable add ( string_value_ clob ) ;
update jbpm4_variable set string_value_ = string_value_2;
commit;