c# sizeof decimal?

The decimal keyword indicates a 128-bit data type.

Source: MSDN

As others have said, decimal is always 16 bytes (128 bits). The precision of decimal is always 28/29 digits. It's a floating point type, unlike SQL's DECIMAL type. See my article on it for more details.

The size of a System.Decimal in the .NET Framework is fixed at 16 Bytes.