1. I really doesn't matter. If you do it in the fall, the time to do it is after the leaves drop which means the tree has gone dormant. In the Spring you move it before the buds on trees start to swell and leaf open. From what I've read in the past, Magnolia trees do a little better if moved in the spring.

The rule of thumb is for every 1 inch in diameter of trunk, you should dig the root ball at least 12 inches wide and almost as deep. So if the trunk is 4 inches wide then the root ball should be at least 48 inches wide and be careful not to crack or break up the root ball.
When you plant the tree the hole should be dug or tilled in an area that's 3 to 5 times as wide as the root ball of the tree and dig it the same depth as the root ball is deep, so don't dig deeper than the root ball. Planting it in wide but not deep hole will allow better root establishment which will help with growth and health of the tree. Fill it back in with the soil you dug out while you are lightly watering it and packing it firmly with your hands (Don't stomp the soil).
Finally mulch the wide area with 3 to 4 inches of organic mulch, (and keep it off the trunk) keep it watered good though the next couple of years until it establishes well.

2. Its root should not have spread too far maybe 1 foot or so outwards in all directions.

3. Most of the time roots will not bother a pipe if they are not cracked open. If a tree is planted right by them it might affect the pipes after many years but I'm not very sure.

Thank you for your reply! Good info. I will have to decide when to move it.
As far as moving it in the fall, Magnolias usually don't lose their leaves. Is there another way to tell that the tree is dormant?

Thank you!

Some magnolias types are deciduous and lose their leaves in the fall and some are evergreen and keep their leaves through winter. I think the reason it has been suggested before by some to move them in the spring is because cold freezing temperatures to evergreen magnolia leaves can become frost burned if they are stressed badly in the cold winter after a transplant in the fall. However deciduous magnolias are fine to move in fall or spring because they lose their leaves in the fall. It also needs to be noted that Transplant shock is common with magnolias and they are often kind of difficult to move because of their wide spreading fleshy root system, so digging a bigger root ball then necessary can give a better chance of success. Putting the root ball in a tarp to move it can hold the root ball together which will help your tree a whole lot by not disturbing the roots in the root ball. You can tell if your evergreen magnolia is dormant because in the fall deciduous trees in your area will lose there leaves and that's when they will go dormant also. In the spring you can tell if they are dormant because trees in the area will have no leaves when they start to bud out or you magnolia starts to put out new green growth that's when they are coming out of dormancy.