Oh wow - thanks matt. dont go to too much trouble becuase i wont have the cash for it for another couple of months.
The box size - doesnt really matter. Any normal size computer box would do. Probably a similar footprint to a normal desktop computer laying down.
Cost - as cheap as possible. Like I said, i dont really want it for paying games or anything, maily just so i can use my tv screen as a computer screen, watch youtube and movies off the computer, maybe tune it into live tv and record onto the computer. I know it will probably around $1000 or more, thats why i thought maybe getting an old desktop computer and upgrading. Im sure i can find an old desktop laying around the house or work somewhere. Ultimatley id like something around $500 that will do most of these goals....(hopefully)