I agree with you. Also I can recommend to go to the special shops and try different types of mattresses(latex, foam, etc). Then you will know what you need. If you want to find some good information about KingsDown models I recommend to read this review from former mattress salesman.
I would say the size depends on the size of the people. A full sized mattress is fine for 2 people that are average size and weight . Personally, I love gig mattresses. I used to sleep on double, queen and now have a king size bed. I wouldn't have a smaller bed now.