Thanks, this works… I altered a little bit so it animated only the width not height and made use of ease.
val = footage("value.json").dataValue([0])
rect = thisComp.layer("Null 1").effect("Slider Control")("Slider");
[ease(time,rect.key(1).time, rect.key(2).time, rect.key(1).value, val),20]
Can you show the alternative way you said of getting rid of the slider?