Hi Stella,
Thank you for contacting Syncfusion support.
We have prepared a sample and tried to meet your requirement. By default, the CenterButtonAnimation property of SfRadialMenu makes the center button size bigger when the menu is opened and shrink when it closed. We can even make the center button size bigger when opening the menu by using Opening event of SfRadialMenu and smaller when closing the menu by using Closing event of SfRadialMenu. Please find the sample from the link given below.
Please update us, if you need any assistance on this.
Regards,
Paul Anderson