ignore those weight limits. you will do just fine on a 156. They put those up there cause for some reason they think that like height, your weight automatically determines your board size. The board will be nice and flexy too =)
Edit: Let me rephrase, YOU at your weight should ignore those limits, 190 is not really that heavy, however someone at 250 lbs prob should pay a little attention to the limits and not buy a 148 or something.
I'm sorry, this is the single worst advice next to people sizing snowboarders based on height. Weight is the MAJOR
determinant when choosing a board.
With that said, the weight limits are there for a very good reason. The intended flex of the board is designed for people within that weight range. However, what I think my good friend is trying to say is that you shouldn't take those weight suggestions as gospel. They are meant to give you a starting point. From a 163, you can go lower for more flex and control, or go higher for less flex and less control, but more float and speed. You also need to realize that some boards are inherently "flexy" or as people call them... Noodles. These are mainly park boards. You don't want to go too low if the board is already flexy for your suggested size.
Another factor you need to consider is your riding style. Are you in the park most? Then go with a smaller board for more control. Do you ride powder (fresh, natural, fluffy snow)? Then go with a longer board for better float. In the powder situation, a 190lb person on a 156 board is going to have his back leg burning like a mofo by the end of the run even if it is set-back. Do you ride all-mountain? Then the suggested weight is great for that type + or - 2cm.
Now, with a Rocker board, you can definitely go lower than normal and still have excellent powder performance. This is due to the shape of the board that naturally gives you more float and control.
I am 5'9 (height does not matter unless you are extremely abnormal like 7'5 100lbs), 175-180lbs, size 9.5 boots and I ride a 158. I am a freerider that likes to do flatland tricks. I don't do park. If I rode park, I'd hop on a 155 or 156.
I tried my friends Custom X (no rocker) 156cm, and I did not like it. I had a lot of control, but it was too slow for me. That's another factor with longer boards. You get more speed.
I'm not sure what size options are available to you, but I think a 157 or 158 would suit you really well.