Let the length be 'l', width be 'w', and perimeter be 'p'.

We have:

lw = 1,000,000.

And p = 2l + 2w.

We can re-arrange the first:

w = 1,000,000/l.

And substitute in the second:

p = 2l + 2,000,000/l.

Finding dp/dl and letting it equal 0 will give you a minimum length. Can you continue?