A SIMD instruction can operate on multiple data elements in one single instruction. It uses the so-called 128-bit SIMD floating-point register. In this sense, vectorization adds some form of parallelism since one instruction is applied to many parts of say a vector.
The number of elements which can be operated on in parallel range from four single-precision floating point data elements in so-called Streaming SIMD Extensions and two double-precision floating-point data elements in Streaming SIMD Extensions 2 to sixteen byte operations in a 128-bit register in Streaming SIMD Extensions 2. Thus, vector-length ranges from 2 to 16, depending on the instruction extensions used and on the data type.
IN summary, our instructions operate on 128 bit (16 byte) operands