Hi Anis,
Thanks for contacting Syncfusion support.
We have looked into your query “Resize the width of a GroupButton” and your screenshot. We recommend using the width property as “%” while having many numbers of data in your application. Also, we have faced the same issue while having much numbers of list data as a width in “%”. Therefore, we have logged this issue at our end. It will be included in our 2018 Volume 1 Service Pack 1 release which will be available by end of March 2018.
Regards,
Sureshkumar P