If you get a 75 it will be 4 feet long, and 18 inches from the back of the tank to the front of the tank. It is I think 21" deep, from top to bottom.
You would want a 4ft fixture on a 4ft tank, if you go smaller you will have a dark spot on either end of your tank.
The spectrum depends on what you like. I have 2 whites ( 12000k) and 2 blues on my 48" fixture. The higher the number the bluer it gets. I tend to like a blue-ish hued tank myself. they make purple bulbs too that you can mix in and get a different hue.
My 125g has 6 10000k bulbs and 6 blue bulbs. I find the color to be too "white" for me.
180g Reef - 60g Rimless Reef -90G FOWLR- 125g Malawi Cichlids- 40b REEF- 34g Fresh Planted-working on- 20L FOWLR- working on
I have a mix of many different saltwater fish amongst my tanks, but I love my Tangs most of all.