I need to install A new air compressor in an area that has the power maxed out. Can I feed off of A 600amp sub station breaker to a 200amp fused disconnect what will my conductors be rated at 600 or 200 the distance is about 40 feet.

If your substation breaker is presently a spare, then the conductors need to be good for 600A, paralell 350KcMil will do the job. Now terminating them in a 200A disconnect might be a problem. But you could tap the 350's with 3/0 for terminations. Check Art. 240-21 for the appropriate application to your situation.
Hope this helps.

Especialy if there is already a lack of distribution, then install a 600A panelboard/switchboard, and take a 200A breaker from there. Next project, you'll be sorry you wasted a big breaker for a small load.

Thank you for the ideas. I have A hard time convincing the boss that we need to think about the future. He only worries about how much this project costs not the problems it will cause for others down the road. thanks again