(ISTAT('RX1SIZE'))
The RX1SIZE parameter returns the size, in bytes, of the RX1 file of the currently connected database. You may need to use the following syntax for large databases:
SET VAR vSize DOUBLE = (ISTAT('RX1SIZE'))
This is simply because the default data type, INTEGER, may not be able to hold values as large as you need.
For a combined size use the DBSIZE parameter.