Why do you call it dot product? That's a well defined function and returns a scalar.
Please place typical values into your controls, make the current values default, save the VI and reattach it here.
There are many matrix operations that create a matrix out of two 1D arrays (e.g. the "outer product".)
What is the size of M? One array (x) seems to be about 8M(!) How big do you expect the resulting matrix to be?