How to make watermark transparent and reduce image size using scale2ref in ffmpeg?

Question

fmpeg

trying to watermark a video

ffmpeg -y -i shark.mp4 -i watermark1.jpg -filter_complex "[1]format=bgra,colorchannelmixer=aa=0.4,rotate=0:[email protected]:ow=rotw(0):oh=roth(0)[image1];[0][image1]overlay=(main_w-overlay_w)/2:(main_h-overlay_h)/2" output2.mp4

so the picture is superimposed in the center, but it is still necessary to make the picture decrease, in one script I saw this scale2ref=iw*0.4:-1, but where to add this function?