Oracle SQLPlus setting environment variable based on variable

Question

I want to set the environment variable long based on the size of the XML data I'm trying to retrieve. The idea is something like this:

var XML_DATA_SIZE number;

SELECT TRIM(LENGTH(xmltype.getClobVal(xml_data))) 
  INTO :XML_data_size 
  FROM xml_tab 
 WHERE key = '1234';

print XML_DATA_SIZE
set long XML_DATA_SIZE
set pagesize 0
set line 2000

set termout off
spool XMLDATA.xml
select xml_data from xml_tab where key = '1234';
spool off

This yields an error: SP2-0268: long option not a valid number, and the XML file only contains 80 characters.