If I have a 200 amp disconnect feed by a 100 amp breaker what size wire do I need on the load side of the disconnect?

Conductors with at least 100A ampacity.

Why do you have a 200A disc on the load side of a 100A breaker?

Don't know. One line does not even show a disconnect for this dimmer panel but it came out with a 200 amp disconnect and 3/0 wire. I had to mount it further than the wire is long that they sent out and was wondering why do I need this wire oversized and if it had to be sized for the bresker in the disconnect or the line side 100 amp breaker.

Dimmer panel? 3/0 for 200A?

This is not in your home, is it?

If this is your job then I would ask the people send you to do it.

Good point stanfortyman