When buying tires for a Motorcycle can you adjust the size along as it fits the same rim size?
Some. You have to pay attention to the room you have to fit larger tires on the same rim. If you look closely at the stock tire on the bike, you will see many places where hard parts of the bike are very close the the tire. That's how much room you have to add a larger tire. It's not a problem as long as the tire doesn't rub on something.
Usually not as the clearances around the tyre will not allow that. In any case even a slight size change can affect the handling a lot so it is not advisable.
Not if you wish your bike to handle and control as the engineers and designers of the motorcycle had intended. They spent a ton of time, money and energy determining what tire size would be best, you should trust their findings.
