Here's my veg chamber, kind of makeshift but works well enough. It's 36"x18"x20" high. The lights are 60w replacement LEDs, 8.5w draw. Guess I should put more mylar on that top part, just didn't get around to it, had to plug a light hole with that cardboard for now. Anyway, the tops are very healthy but there is some lower leaf drop, but the canopies are pretty much closed over so not surprising. I was using 14w bulbs before but too much bleaching so had to dowsnsize to the 8.5s. That's two of the three lights there.
I guess the thing with the low wattage is shallow penetration. It was better with the 14w bulbs but as I said, hard on the tops. I find 4 plants per light works well. I mounted the lights through holes so only the 1/2" of diffuser is inside the chamber and there's some air space around it for air to chimney through and cool them somewhat. I needed all the vertical space I could get and mounting them inside ate up too much and was also too hot in there. Those plants are 12" from pot to top, ready to flower. Actually I give them the first week of flowering right in that chamber, keeps them manageably small and makes a 10 week flower period possible in a perpetual with 3 larger chambers at 3 week intervals.