This is what I found - the data that the PI exports is pretty much unlimited - in the PI_PARTIC_EXPT table, the EXPORT_ROW field is 250 characters. If the row you're exporting exceeds this, a new row is inserted with a new sequence number (export_seq), and the data is continued in the EXPORT_ROW field in this new row.
There is,however, a limit to an idividual field that the PI can handle, and that is 240 characters, so once I limited the field to 240 characters all was well.