I wrote one out on here a few months ago. Have you done a search?
What do you want to put in for lights?
If you go with MH, make it at least 12" tall.
I made mine 51" long. 48" bulbs fit in nicely.
It is also 13" deep for room.
Across the top I have wood strips instead of a solid board for heat to escape.
Just some ideas to think about. Try a search, if it doesn't help post back.
HTH