Take a look at the screen shot in the poor guys post for example. As you can see, this has been an issue for many developers for years now.
The only known way to reduce the billboard size is to reduce the width of the control point. This is not a solution though as doing this directly affects the material that is applied to the spline. It becomes a real problem if you need to move the camera in close to a control point to make fine adjustments. The billboard just ends up filling the entire screen as it does not scale with camera distance.
I think two things need to happen here.
The billboard needs to scale with camera distance… so as the camera gets closer, it gets smaller. It should stay the same relative size on the screen at all times.
There should be a Billboard Scale (percentage) field on the control points that feeds into the above equation, so the relative size can be adjusted per control point if needed.
If the first point is too much work, then at least just expose a billboard scale like in the second point.