If you must know the runtime size of a data sequence (that is, the number of nodes or data items in the sequence), you can get it by using the combination of the XPath built-in count() function and the BPEL built-in getVariableData() function.

The code in the following example calculates the number of elements in the item sequence and assigns it to the integer variable lineItemSize.

<assign>
   <copy>
      <from expression="count(bpws:getVariableData('outpoint', 'payload',
                        '/p:invoice/p:lineItems/p:item')"/>
      <to variable="lineItemSize"/>
   </copy>
</assign>