Hi,
the step size only controls by how much the brightness is increased/decreased if you send a IncreaseDecreaseCommand to the thing. It is currently hardcoded to 30. This means that if you send an IncreaseCommand, the brightness is increased by 30 (hue brightness is internally represented as values between 1 and 254).
I guess this is not what you’re looking for, though. What you need is to control the transition time, which is currently not possible with the binding. This is the related ticket.
As the workaround to control the transition time you can make a manual request to the hue api, as mentioned in this post.