Hello,
You can change group layout (so also dial sizes) in the "Edit group" window - right click group name or click the main menu to access "Edit group".
See also introduction video:
https://www.youtube.com/watch?v=mgAJTjrcIRI
I'm also working on a updated documentation:
https://github.com/fastaddons/GroupSpeedDial/wiki/Group-layouts
Best Regards,
Juraj Mäsiar