Rolf Tesmer has a nice series on partitioning going. His latest entry involves calculating partition sizes in advance:
Sometimes (just sometimes) you need to calculate the size your table partitions upfrontbefore you actually go to the pain and effort of partitioning (or repartition) a table. Doing this helps with pre-sizing the database files in advance instead of having them auto-grow many many times over in small increments as you cut data over into the partitions.
Check out the entire series.