Particle effects - decreasing size over time

I'm trying to have an image (that is being emitted) decrease in size over time. I'm able to set visibility over time, speed over time etc. but whenever I try to change the size nothing happens. It stays the same size as the last value/point in the timeline/field. So I'm guessing there's some option that stops me from being able to scale it over time? If not, does this have to be configured in a specific way?