Choosing a snowboard is an important task that requires some time and research. Firstly you need to know your height and your weight, your riding level and the types of terrain you are likely to be riding.
Many riders will prefer to have 2 or 3 boards for different conditions. For example I am 5′41/2” And within the average weight range for my size. I would ideally own 3 boards a 148 for jibs, tricks and hard terrain. A 152 for average conditions all mountain riding and a 156 for those deep deep powder days. In many circumstances it is not convenient to carry 3 boards everywhere, check the conditions and style that you are likely to be riding.
So after you have considered these questions you will have to work out what is best for you. For the beginners out there I would probably go with the mid size board for all mountain riding. Some hire shops and retailers will offer you a board that, when stood on it’s end, comes between your chin and nose. Don’t forget to check your weight, if you are above or below average for your height you will need a longer or shorter board. Most boards will have a weight chart stuck onto them in the shops, it is worth going and having a look at some. Remember to take into account any equipment that you may carry with you as this will add to your weight, particularly if your considering back country riding.
The other factor that needs to be checked out is your foot size, when you stand on a board in your normal stance you should be aware of overhang at the front and back of the board, toe and heel drag can cause no end of problems for your riding style. You should have no more than 1/2” overhang, if there is more than that look for a wider board.
Choosing a snowboard is not a straight forward task, make sure you take your time and do as much research as you can, it will be worth it. I have provided some useful links below for more information.
http://www.theboarderline.com/lengths.html
http://www.burton.com/LTR/LTR_new_rider_info.aspx
http://www.abc-of-snowboarding.com/buying-guide/buyingsnowboards.asp
http://www.ridesnowboards.com/boardfinder/
Technical info on board structures