Here are some general tips.
Where your nose is, does not determine what size of snowboard you should ride!
...or your chin, ears, shoulders or any other body part for that matter. These are the silliest rules for sizing boards that could possibly be imagined, and yet they persist. We hear new ones everyday, "my friend told me that a board should come to in between my chin and my nose." Why, are you planning to nibble on it? These generalities are good ways to end up with a completely inappropriate board. Why do such rules exist, you ask? It is due to the fact that finding the right board takes a bit of research and knowledge. The easy way, however incorrect, is much quicker. A snowboard reacts to only two factors, how much pressure is being applied to it (weight), and where that pressure is coming from (shoe size). Boards are designed around riders of a certain weight. The total weight range for a given board will be around 50 pounds (although manufacturers tend to exaggerate this range to make their products sellable to a wider variety of customers). Two men who stand six feet tall and have their noses at identical heights, may be separated by 100 pounds of weight. This would change the boards they should ride by two entire categories of stiffness, and length. You will also want to make sure that the board is appropriate for your shoe size. One half to three quarters of an inch of overhang (yes, overhang) off the edge of your board is ideal (when wearing snowboard boots, and measured at the stance angle that you will ride). We will discuss this more below when we address width in detail.
There is no best level of stiffness for a board!
At least five times a day we hear,"the guy at mountain told me that I want a soft board." This is the part that we were discussing above that relates to weight. Snowboards react to pressure that is applied to that hourglass shape (sidecut) that they have. This shape, when flexed, creates an arc on the snow. You are planning on turning on that arc. If you can't flex the sidecut into the snow (because the board is too stiff for you) you simply can't turn well, or not at all. If the board is too soft for your weight, it will constantly be overflexing, and "twisting off" of the edge that you are relying on to carve. In this scenario you will have a terrible time on hardpack and ice, because the "effective edge" (amount of edge that should be in contact with the snow) will be twisted out of shape, and not doing it's job. Softer flexing boards tend to be better for lighter riders, while stiffer boards are needed for the big boys. Only for extreme freestyle, or extreme race applications, should this rule be broken (and in those instances, a second board will be needed for all mountain riding).
Buying by length is the hardest way to end up with the right board!
"My last board was a 156, and I liked it, so tell me about the 156's that you carry." The trick here, is that two boards of identical length, may be designed for completely different riders and types of riding. For example a 156 may be a "big mountain board" for a small woman, or a "park" board for a big guy, depending on the manufacturer's design plan. Those two boards, however, would never be appropriate for the same rider. Length is often discussed in terms of: longer equals faster, and more stable, while shorter equals more maneuverable. This can also be deceptive. The "running surface" of a board (the base area that contacts the snow) is a useful measurement, because this is the amount of board that you actually are riding upon. The overall length (the measurement usually considered) can be misleading, as it also contains the raised tip and tail, which do not contact the snow, and have only nuance differences in affecting your ride. Your best bet is research. Look into who the board was made for, and for what type of riding. Leave the rules of thumb to the rental guys, who are trying to get through the line of renters as quickly as possible, and get on the slopes (can't blame 'em for that).
As for width, we recently did a blog post on that. I hope it is helpful:
The Boarder's Blog - Snowboard Width - Huh?