Generally, the answer is no; you won't feel height sick in a balloon, even if you are afraid of heights. This is a difficult one to explain why. Strangely enough, from a balloon basket, no matter how high you are flying, you do not have the feeling of depth that you have, for example, if you are looking over the edge of a tall building. It constantly surprises passengers how safe they feel once in the air, even though they thought they would be a little scared before the flight. You can lean on the side of the basket quite comfortably and enjoy the views with a feeling of safety; no need to worry.