I just bought a Rome Agent as all mountain board, it gets great reviews here. I received it but I have not yet tried it so I can not tell you more about it. But many members here recommeded the board for all mountain riding with great park capabilities. Do a search for it.
As far as the size, it depends on what you like. I tend to buy boards at the shorter end of my range, as I like to have more control over my board than high speed. If you are planning to him more powder I would recommend to buy a longer board, but if you are more orientated towards park go towards the shorter board.