When pasting a big image in RichTextBox, it is pasted with its original size. It should be resized so it can fit on the page.

Similar logic is available in the InsertPictureCommand.

Workaround:

    private void RadRichTextBox_CommandExecuting(object sender, CommandExecutingEventArgs e)
        {
            if (!(e.Command is PasteCommand))
            {
                return;
            }

            DocumentFragment res = ClipboardEx.GetDocumentFromClipboard("RadDocumentGUID");
            if (res == null)
            {
                foreach (ClipboardHandler settings in ClipboardEx.ClipboardHandlers)
                {
                    res = ClipboardEx.GetDocumentFromClipboard(settings.ClipboardDataFormat, settings.ClipboardStringFilter);
                }
            }

            if (res == null)
            {
                e.Cancel = true;
                var bitmapSource = Clipboard.GetImage();
                if (bitmapSource == null)
                {
                    return;
                }

                Padding sectionmargin = this.radRichTextBox.Document.CaretPosition.GetCurrentSectionBox().AssociatedSection.ActualPageMargin;

                double originalPixelWidth = bitmapSource.Width;
                double originalPixelHeight = bitmapSource.Height;

                if (originalPixelWidth == 0 || originalPixelHeight == 0)
                {
                    originalPixelWidth = 10;
                    originalPixelHeight = 10;
                }

                double width = originalPixelWidth;
                double height = originalPixelHeight;

                if (this.radRichTextBox.Document.LayoutMode == DocumentLayoutMode.Paged)
                {
                    Section currentSection = this.radRichTextBox.Document.CaretPosition.GetCurrentSectionBox().AssociatedSection;
                    var pageSize = (SizeF) currentSection.GetType().GetProperty("ActualPageSize", System.Reflection.BindingFlags.Instance | System.Reflection.BindingFlags.NonPublic).GetValue(currentSection, null);

                    double maxWidth = pageSize.Width - (sectionmargin.Left + sectionmargin.Right);
                    double maxHeight = pageSize.Height - (sectionmargin.Top + sectionmargin.Bottom);
                    width = Math.Min(maxWidth, width);
                    height = Math.Min(maxHeight, height);
                }

                double ratio = originalPixelWidth / originalPixelHeight;
                width = Math.Min(width, height * ratio);
                height = width / ratio;

                Size size = new Size(width, height);
                ImageInline imageInline = new ImageInline(new WriteableBitmap(bitmapSource));
                imageInline.Size = size;

                this.radRichTextBox.ActiveDocumentEditor.InsertInline(imageInline);
            }
        }