Long vs Short

A snowboard should typically be as tall as your chin is from the ground, but sometimes people like them either a little shorter or a little taller. The shorter your snowboard is, the slower you will go down the mountain; thus the longer your snowboard is, the faster you will go down the mountain.
A smaller snowboard would be more preferable in the terrain park because it's easier to control when grinding on rails and going off jumps. On the other hand, a longer snowboard would be more preferable when "cruising" down the mountain. A longer snowboard not only gives you more speed, but it's also easier to control, it gives you a smoother ride, and you are less likely to "bite an edge". Biting an edge would be when your board digs into the snow, making you fall suddenly and very hard to say the least.
The size of a snowboard is completely up to your personal preference, some boards just handle better on different terrains.

Goofy or Regular

There are two different types of stances you can have when snowboarding, and it's all determinded on which foot is your dominant foot. If you ride down the mountain with your left foot guiding you, meaning that it is mostly infront, then you are considered to have a regular stance. On the other hand if you ride with your right foot guiding you, once again it means that it's mostly infront, then you are considered to have a goofy stance.
The easiest way to determine your stance is with a pair of socks and a hardwood floor. Wearing just the socks on your feet, run and slide on the hardwood floor; the foot that is infront of you while sliding will be your dominant foot while snowboarding.
Your stance will be consistant throughout snowboarding, surfing, skateboarding, and anything else requiring a dominant foot.

Jumps, jumps, jumps!

When learning how to go off jumps on a snowboard there is a few key pointers that can make that goal a lot easier to achieve. The first, and probably most important, part is making sure that you are not going too fast because you can overshoot your landing. Your landing is the part of the jump that declines back down onto the flat surface of the mountain, overshooting this can result in injury.
As you are approaching the jump it’s important to be going head on with the “lip” of the jump, going off the jump at an angle can result in you going off the side or spinning in the air. This can be something you want to do once you conquer the basic jump, but for starters it’s best to just go straight off the jump. Going off the jump straight also makes it easier to be ready for your landing, which also requires you to be straight on with the bottom of the mountain. If you land at an angle you will most likely “bite an edge”, which is where the side of your board digs into the snow and you land face first into the ground. Trust me this is no fun, but it happens to everyone when learning how to do jumps on their board.
The last thing you want to do is to bend your knees as you land because it braces your impact. You want to support yourself and take the stress of your knees, which you can do by bending your knees, just as you would by bending your knees when jumping off a chair. This will keep you stable and less vulnerable from hurting yourself.
These pointers will help you out when learning how to do jumps, but it’s really a trail and error process. The best way to learn is by getting out there and doing it, so I suggest you get out and do it!