Hi, I'm glad you enjoy RCSpline !
1. Unfortunately it's not possible to change the size of text. Max uses a fixed size font, and is not customizable.
The marker itself would be changed between several available types, same with the color.
If that helps, we use the standard "Gizmos" color, which is actually customizable from Max (Customize->Customize User Interface->Color->Gizmos->Gizmos).
2) Agreed. Added to the wishlist.
3) I discussed this issue with our team for a while: i see the point to use a customizable data list for User Data, instead numeric values. But the problem is where to store that customization.
Assuming you want to reuse it, it has no sense to customize the list each time you add a RCSpline modifier.
Ideally the data list should be linked in some way with the RC object, but there is not a simple way to link a Modifier used in a Spline, with a RC Object which uses that Spline.
Also it would require to add a complete system in RC, to edit and save multiple data list for different types of RCSpline modifiers. Everything saved in the RC style... really complex.
We continue evaluating possible options for this feature.
Thanks for your suggestions.