The garage is attached, and it has decent supply, same as the house, Just measured at at 14 foot wide, but don't want to use more then 7 feet in width, lenght is less of an issue. but i don't want to go too extreme. Let's say for the sake of clearer answers that I make a box 6x5. Do you think 1000w is too much?

Also i'll check out the hydrohut deal, thanks for the heads up.