This video was uploaded by yesheflowers. Be sure to read more about how to sign "waterfall" below the video.
Signing Savvy demonstrates how to sign one version of "waterfall" here:
The proportion of size and movement can easily be shown to differentiate if it is a little waterfall or a huge waterfall. ASL is very expressive, you can show a little waterfall by using gentle movement of your hands in the falling motion, or you can exaggerate the falling movement to show a HUGE waterfall, or just as demonstrated for a moderate waterfall.