Thanks for the debate guys, you've helped give me some ideas.
Final internal dimensions of my grow box are 1126mm W x 563mm D x 1468 H.
Also, I'm going with 1 single 600 Watt HPS.
I'm thinking of having 1 intake fan about half way up the box on one side. Then on the other side near the top i'm thinking of having 2 fans sucking heat out through 100mm tubing and then plumbed together to exit finally through a 100mm x 400mm carbon filter. Now i'm just wondering what size fans I need. Any suggestions would be much appreciated.