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The higher the better. The space above the cabinets accumulates rising heat and minimizes its mixing with cold air. This improves air conditioning efficiency. Obviously, it also provides space for overhead cable trays and good lighting. We generally like to see at least 12 feet, but 14 feet is even better.
The only drawback to very high ceilings is the cost of inert gas fire suppression, if you are using it. (With sprinklers, it doesn't matter.) Since the amount of gas needed is based on the volume of the room, high ceilings add to its cost, so there's a tradeoff involved. With gas fire suppression, I probably wouldn't go over 14-feet. We have done data centers with as little as 9' ceilings, but if it's this low, it's best if you can duct the return air back to the air conditioners from each hot aisle. If your ceiling is this low, it is also probable that you will have a fairly low raised floor (assuming you're using under-floor air distribution). If you can't get at least an 18" floor height, you're going to significantly limit how much wattage you can put in a cabinet. If the data center is small as well, you'll also have rather uneven air flow. Engineering is a business of tradeoffs, and you should have a careful evaluation, including CFD (Computerized Fluid Dynamics) modeling, before deciding on floor and ceiling heights in a restricted space.
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