2 indica plants can easily grow in a 2x2, 4 feet high. (If possible allowing more height just incase you get a stretcher).
In a 2x2 with 2 Indica plants you could look to yield about 4-6 ounces. You would need 1 month for veg, 2 months for flower, 1 month for dry/cure. So 4-6 ounces every 4 months.
In a 2x2 area I would recommend 250watt HPS/MH ballast. Digital ballasts can run both HPS and MH bulbs.
250 watts will put off a lot of heat, that becomes a factor. You will want to keep your grow room between 74-84 during lights on with a 10 degree + drop at night during flower. That will require a fan sucking the heat off the lightbulb and expelling it outside of the grow area. For my 400 watt light, I use a inline duct booster fan from homedepot for 30 dollars. If it will work for a 400 watt, it would certainly work for a 250 watt.
As far as it goes, 400 watt light isnt much more than a 250 watt light, and if you decide to enlarge your area to 3x3, now or in the future, then you will have enough light for that as well.