I don't suggest you do a wide board with only 10.5 boots. Not even at the size 157. If all you ride is powder, then a wide board will be great. For your weight and riding style, I recommend you ride a 159 or 160 regular.
Here are the Pros and Cons with board lengths...
Shorter Boards (boards that are below your weight range)
Pros: More control, more flex (even if the board is meant to be a little stiffer, since you are well above the weight range of the board, you are effectively making it softer), much easier to perform tricks on
Cons: Less float in soft snow such as powder, generally less speed (speed is highly dependent on snow conditions and rider skill level0), less stability at higher speeds, less "pop" - the reason for the less pop has to do with the above explanation of you being over the weight range for the board
Longer Boards (boards that are your weight range and above)
Pros: More float especially good for powder riding, more stability at speed, more "pop", board is designed with your weight range in mind so you get the intended feel of the board, generally more speed
Cons: Less control than shorter boards, are slightly heavier, harder to do tricks like spins
There are probably more to this, but it is all I can think of at the moment. I'm sure others will add their expertise.
Of course, a rockered board will change all of these factors. For example, if you get a 157 snowboard with rocker, you will actually gain float. Still, a 160 rocker will float better than a 157 rocker. Equally so, a 160 rocker will make it easier to butter and control, but a 157 rocker will still be better at it.
Now, you can ride a 157 just fine for all-mountain at your weight. You will have to adjust your riding to accommodate for the smaller size. For example, let's say you are riding in powder with this 157. You will most likely have to set your bindings back on the board and will have to use your back leg more to keep your nose above the pow. You'll be able to ride it, but at the end of the day your leg will hate you for it.
If you want to go with a 157, try one with a rocker. A 158 with rocker will be even better. Avoid the wide boards, you don't need those unless your boot size is 12 +