Tank Size?

[Vintage]

Hey guys, I am debating which size I will make my tank.The light fixture that I already have made is 10" long, with 5 3w Cree LED's (3RB 2CW). They are set up in a row and are spread out over approx. 9". So my question is, how big of a tank can I make allowing for proper light coverage and good coral growth? The only corals I know I want to keep are GSP and Frogspawn, other than that I'm pretty flexible.

(The Light was made for my 2.5 gallon, so if I must, I will just use that, but I want to make my own for two reasons, More room and more optimal dimensions.)

Thanks guys, I feel bad for asking so many questions, but I want to plan this out right!