Internally, the Decimal is a 17-byte number that can contain any decimal number with up to 16 digits to the left of the decimal point and 16 digits to the right of the decimal point. The external interface is as a SQLNUMERIC_STRUCT as defined in ODBC 1.0.
-9999999999999999.9999999999999999 to 9999999999999999.9999999999999999 (-9,999,999,999,999,999.9999999999999999 to 9,999,999,999,999,999.9999999999999999)
For more information, refer to Declaring Variables and the Decimal command.