So perform the experiment yourself:

Console.WriteLine(sizeof(decimal) * 8);

Eq.

This is from the MSDN Library (so you can believe it, rather than something
you "read somewhere"):

"The binary representation of a Decimal value consists of a 1-bit sign, a
96-bit integer number, and a scaling factor used to divide the 96-bit
integer and specify what portion of it is a decimal fraction. The scaling
factor is implicitly the number 10, raised to an exponent ranging from 0 to
28. "

Decimal uses 96 bits to store the mantissa, 5 bits to store the
exponent, and 1 bit to store the sign. However, it's stored as a 128
bit value. So yes, 26 bits are wasted - but it would be hard to put
them to good use.