you need anywhere from 35 to 50 watts per square foot. a 400 watt hps or metal halide will cover 8 square feet very well.

I'm guessing that your little plant collection will cover about 20-30 square feet. 400W should be enough just to overwinter them. Keep it on the cool side so they don't grow too much. If you want to have them growing over the winter then I think you will need 1000W.

Mine grow over winter and I use 1000W. My trees are a tad smaller than yours but I also have smaller low-light plants around the perimeter of the citrus.

I use MH not HPS but I get no natural light, and really HPS should work fine as well.

Although, right now my MH light is not working and my poor trees are sitting in the dark. I figure they were in the dark for 2 days when they were shipped to me so they should survive so long as I fix it sooner than later!!

I had to spring for a new ballast. But now my light burns noticeably brighter than before the old one went out.