Question
I want to move a cottonwood tree that is greater than six feet in diameter at its trunk. Are you laughing at me? Cause this sounds funny to me too. Just wondering if it is possible and what the odds are that the tree would survive. Thank you for your consideration in this matter, Brandie

Answer
I am assuming that the six feet is the width of the foliage not the trunk diameter??!!
Large tree can be move BUT it is a huge job and they weight quite a bit.
Deciduous plants may be moved in the spring as soon as the frost is out of the ground, up until the time when new foliage is partly unfurled. In the fall, they may be planted once the leaves start to turn color up until the ground freezes.

Digging the root ball:

When you want to dig up a tree or shrub for transplanting, retain as much of the root system as possible.

Older deciduous trees can be successfully moved only if a ball of soil is left around the roots. The exposed roots should be protected with moist burlap or newspaper or with polyethylene sheeting. Every effort should be made to reduce root exposure to wind and sun, keeping the ball as moist as possible. It's best to prepare the hole before digging up the tree you wish to move.

Size of the root ball and size of the hole:

For deciduous trees and shrubs the soil ball should be:

Width = 9-12 in. in diameter/every 1 in. of tree diameter
Depth = 6 in./every 1 in. of tree diameter

For example: A tree trunk diameter  4 inches wide would need a soil ball of 36-48 inches wide and 24 inches deep.

The new hole needs to be twice the size of the root ball and as deep. Fill with good top soil or potting soil and mulch around the tree with not more than 3 inches of organic mulch and water good. I would water every third day for the first couple of months and then once a week through the first growing season. Water with 1 inch of water each time (place a pan under the tree and turn the sprinkler on and when the pan has 1 inch of water in it stop).
The  success rate will depend on the root damage and the watering. Cottonwood will do much better than most hardwood trees. You can even with a good root system expect some dieback for the stress of the move.

It will be a big job but it is possible.