Greg Baethge
Ansys Employee

Thank you for posting your question. Just to clarify, could you specify which example you are referring to? We have multiple grating coupler examples on our website.
In any case, as you noted, if you change the length of the taper, this changes the start of the grating, hence it changes the radius of each etched area. The changes you can do will mostly depend on what you want to achieve. If you want to optimize the grating, I would suggest to look at the inverse design optimization examples, starting with the 2D one:
Inverse design of a grating coupler (2D)
Inverse design of a grating coupler (3D)
We typically start with a 2D version as it is much faster. This allows to get a good starting point for the next step, the 3D optimization.
I hope this will help.