Hey 10000_fists, welcome to Boardworld!
Firstly, most workers in rental departments at the snow don't know much about what they're doing. I hate to be negative but I've seen it with my own eyes on many occasions. They see so many people come in and out with snowboards, they really don't have the time, care, or knowledge to ask the right questions. They don't take the time to make sure you get on the best board for your weight, experience, and riding style. The reason you were probably given such a big board when you hired is because you are tall. They use height as a guide to determine what board size you should be on. However this is the wrong way to determine board size. Weight is much more of a factor than height.
That said, every snowboarder can comfortably ride a certain range of snowboard sizes. Each size will offer a particular advantage and disadvantage, depending on their riding style and the snow conditions.
Generally speaking, a longer snowboard is better for freeriding (carving, riding natural terrain, riding fast, powder etc...). A longer board offers more stability; you will notice this mostly when riding at speeds, carving, landing big jumps, or riding powder. A longer board also offers more float in powder (board shape will also make a difference). A disadvantage of a longer board is the fact it is less maneuverable than a shorter snowboard.
A shorter board is more maneuverable, but less stable than a longer board. A shorter board is easier/quicker to turn, easier to spin, and easier to press. As a snowboarder progresses, it is likely they will want to learn freestyle at some point. Shorter boards are really fun for freestyle, which is why most people would suggest you try something shorter.
You need to honestly ask yourself a few questions...
Where will you be riding mainly? (Australian hardpack or deep Japan powder etc...)
What is your riding style? What aspects of snowboarding do you really enjoy? (i.e. freeriding, powder, park, or a bit of everything etc...)
What is your current ability? How many days a season do you plan on riding?
If you can answer those questions, it will be easier to decide which board and size would best suit you.
Have a read of the
Board Selection article and the
Board Characteristics article on Boardworld. There is a lot of good information in there.
I hope I cleared a few things up for you. If you have any questions at all, please let me know. I hope you have a great season!
