First, for Full HD, you must check if the TV has 1920 x 1080 resolution. The smallest size available now in the market is 37 inches. But Sharp will have a 32 inches coming later. For brightness and contrast, the higher the better. Higher brightness allow you to watch the picture clearly even with strong room light or even sunlight. Higher contrast allow a better picture quality so that colors can be differentiated between each other even with very little difference. But since manufacturers have their own way to measure, these figures cannot be very good comparison. So see with your own eyes to compare. The response time is another thing that you must check. At least 8ms is OK, the lesser the better.
To decide the size you have to buy, just a reference that for 8 ft distance between the TV and the seating place, a 40 to 42 inches TV is OK, 6 ft distance, a 37 inches, 5 ft, 32 inches.