If it be a square tank, the side of tank should be 8.52 m to hold about 619950 litres of water. In case you have to design a cylindrical tank you can find out the radius and height as follows

6,19,950,000 cc= 3.14*r*r*h

Say the height of the tank is equal to radius, then

6,19,950,000 cc=3.14*r^3

r^3=6,19,950,000/3.14

r=582 cm or 5.82 m