Note: other operations that affect the image's size should also handle large images.

No Workaround.

It is possible to create a custom resize dialog as follows:
        public class MyRadImageEditor : RadImageEditor
        {
            protected override RadImageEditorElement CreateImageEditorElement()
            {
                return new MyRadImageEditorElement();
            }
        }

        public class MyRadImageEditorElement : RadImageEditorElement
        {
            public override void ShowResizeDialog()
            {
                this.ShowDialog(typeof(MyResizeDialog));
            }
        }

        public class MyResizeDialog : RadForm
        {
            public MyResizeDialog(RadImageEditorElement imageEditorElement)  
            {
               
            }

            //TODO custom dialog
        }