Graphite's .wsp database files are allocated on the filesystem for the full configured retention the first time the metric comes in. These files are sparse and are 'filled in' as data is written into them. I usually estimate about 5GB per node but the actual required can vary a lot depending on the number of instances (volumes, lifs, luns, etc) you have that are all individually tracked.
If you have the data on a filesystem backed by a NetApp LUN or virtual disk on NetApp you will only have storage consumption on the NetApp as you actually fill the files.
Cheers, Chris Madden
Solution Architect - 3rd Platform - Systems Engineering NetApp EMEA (and author of Harvest)