Hello,
Try a different "Group layout" - first right click a group name, then select "Edit group".
There you can choose between several layout, some allows you to specify number of rows / columns, and in some you can choose the exact size of the tiles.

Some incomplete documentation is also here:
https://github.com/fastaddons/GroupSpeedDial/wiki/Group-layouts
Best Regards,
Juraj Mäsiar