size
From TidalCycles userbase
Type: size :: Pattern Double -> ControlPattern
size controls reverb room decay time. It turns a number pattern (ranging from 0 to 1) into a control pattern that controls the room decay time of the reverb effect.
d1 $ sound "bd" # room "0.5" # size "<0.1 0.5 0.9>"
The room function controls the reverb effect amount.