finding the length of a matrix

[macezz389]

So I'm having troubles with this code. I am working with a huge text file that contains a nx2 matrix. Since "n" can be very large, Matlab can not import the entire file due to memory limits. Therefore, I must import sections of it using 'dlmread'. However, how will I be able to know the size of this huge matrix? I can not just read it off the text file by eye. I need to run a loop to take sections of 100,000 rows at a time, but I must know the number of the last row of the entire matrix to run the loop. Please help.