What product are you using? If DPA, it would be a custom metric.
object_name LIKE '%:Databases%' AND
counter_name = 'Data File(s) Size (KB)' AND
instance_name = '_Total';
Or use whatever you are comfortable. I'd think you could just run this once per day when you set it up, but then it should start plotting size over time.
Was you able to get something working?