The AquaticLife 16.5 inch, 420/460nm Actinic 18 Watt T5 HO Fluorescent Lamps are 16.5 inches from end of pin to end of pin (for 18 inch fixtures/retrofits).
The 420/460nm Actinic bulb spectrum is very beneficial to corals. With reef tanks half your bulbs should be actinic.
The AquaticLife T5 bulbs will work in any T5 fixture, as long as you are using bulbs of the same wattage as those currently in your fixture. T5 bulbs come in two categories, HO bulbs (18W, 24W, 31W, 39W or 54W) and NOs (9W, 14W, 21W or 28W). T5s are 5/8" or .625" in diameter.
