Custom Grow Box Advice

I am curious as to what everyone's opinion is to what works best for size wise for growing 1-2 plants under LED/CFL lighting. I will be lining the inside with mylar with one single built in computer fan for exhaust and one 6" circulation fan inside. Will be using 3 gallon pots for growing Blue Mystic Autoflowers starting with just one and eventually another once I cut in my two 60watt cfl grow bulbs. So whatever anyone thinks works well for that situation, I just need to know height, width, etc. Thanks for all advice!