I have a 200 amp main panel in my house and I just built a garage and am planning on putting a 100 amp sub panel in the garage. The garage is about 200 feet from my house and it has a 2 inch pvc stubbed out underground. I am wondering what size aluminum I would need to feed it, and if I can use a smaller pvc.

I should add that the garage is detached and that I have a 50 amp 240 V welder and a 30 amp 240 V lift and there will be some convenience outlets and lights.

You would need 2/0 aluminum. Your equipment grounding conductor can be 6 AWG aluminum or 8 AWG copper. 2 inch PVC is the correct size for the three 2/0 THWN wires and the grounding wire.

#2cu or 1/0al is all you need. Even that is considering a full time load of 80 amps, which you are likely never to see.

#2cu or 1/0al is all you need. Even that is considering a full time load of 80 amps, which you are likely never to see.

What size ground would I need

Normally a #8 ground would be correct, but since you are increasing the size of the feeder you must also increase the size of the ground.
Without doing all the calculations I think a #6cu would be fine.

Normally a #8 ground would be correct, but since you are increasing the size of the feeder you must also increase the size of the ground.
Without doing all the calculations I think a #6cu would be fine.

Does my neutral need to be the same size as my current caring conductors

Does my neutral need to be the same size as my current caring conductors

If not what size could it be

Having the neutal the same size as the current carying conductors basically means that the entire 200 A at 120 is being fed from one side of the panel.

The neutral can be reduced by the 240 volt loads. e.g a 50 A 240 V load, can be take off 50 A at 120 V since the neutal will be 0 for this load. So, you could deduct 80A @ 120 V from the total. These loads are not continuous, so you can even deduct more.. 1.25 * 80.

You would have to look at the other stuff too.

If you went with the same neutral size, then you could have 100 A of 120 from one leg and 100 A from the other at 120V. With a lower neutral size, the you can't.

The neutral carries the difference of the two hots.