Thank you for using Syncfusion products.
At present there is no option to customize the image size in ChildBarItem and ChildBarItem size will be defined based on its font size. So we suggest you to make use of CustomFontText property in BarItem, to customize its size. Please have the code snippet below which demonstrate the same.
Code Snippet [C#]:
// Font Size
this.barItem1.CustomTextFont = new System.Drawing.Font("Microsoft Sans Serif", 25F, System.Drawing.FontStyle.Regular, System.Drawing.GraphicsUnit.Point, ((byte)(0)));
this.barItem1.ImageSize = new System.Drawing.Size(50, 40);
We have also prepared sample and image for your reference and it can be downloaded from the following location.
Note : The default BarItem image size can be customized using ImageSize property.
Please let us know if it helps,
Thank you for your update.
As per the implementation of child BarItem, its bounds calculation will be done based on its font size. Hence we have suggested you the same. We request you to use this approach, to achieve this reported behavior.
Please let us know if you need any further assistance,