If your matrix is dense and you store floats in it, for example, then your matrix will consume 7.2 gigabytes of space. Are you sure that is what you are trying to do? SymPy matrices will take up even more space than that depending on the variable type.
Can you explain what you are trying to do? There are many options for saving large data to disk but the answer depends on the situation.
If you have a lot of memory on your machine you can try to write the results of srepr() to file or even maybe pickling the matrix.