How to get constant size of vector for an array declaration?

I am reading the data from text file which is basically has rows and columns of double data types. Part of the code is as shown below:

   m_data = vector<vector<double> >(columns, vector<double>(lineCount - 3));
        for (int x = 0; x < lineCount - 3; x++)
        {
            for (int y = 0; y < columns; y++)
            {
                m_data[y][x] = total_Data[it]; //total_Data is the complete data set read from a file 
                it++;                        //which contain all data sets from a simulation and they are 
                                            //separated into vectors here in this code

            }
        }
   size_t len = m_data[1].size();     //m_data[1] vector represents the signal data set
   double signaldata[len];
   copy(m_data[1].begin,m_data[1].end,signaldata);

So the problem is when I copy the data from vector to array it requires the constant length of the array for initialization. But size() gives the length of the vector which is not a constant. And sizeof(vector) returns the size of the object itself which is 16 bytes. So how I can overcome this problem of getting a constant size from vector for array initialization