Hi Kay,
The Answer for your question is yes.The ImageSize in a TreeViewAdv can be set to desired size by setting the size of the image in the ImageList.Because TreeViewAdv control displays the image size depending on the size of the image in the image list.
Please take a look at the sample that illustrates the above and let me know if this helps you.
Thanks for your interest in Syncfusion products.
Regards,
Deepa.
ImageSize.zip