Look at the top of the tank. What is the lenth across the top? This is the lenth of the T-5 lights you want to get. When shopping for T-5 lights, look or ask the place you are dealing with, if the reflectors behind the T-5 bulbs are one reflector for the whole light, or indivial reflectors, bulbs with a indivial reflectors are better. It make better use of the light and sends more of the light down to the tank.
If you are thinking about Metal Halides, then a 150 or 175 watt pendant type light will work well above the tank. (these hang from the ceiling. I would consider a 12, 13 or 14K rated bulb, this will give just a hint of blue to help with the look of the tank.