Table size and table partitioning on a single drive server
Kendra Little just released an excellent article on the topic: How To Decide if You Should Use Table Partitioning.


  This is usually the first question people have:  “Is my table big enough?”  My response is: “Let’s talk about why
  you’re interested in table partitioning. What is the problem you’re
  experiencing?”


Go over the article linked, is very good. It even has pictures of Unicorns! Hopefully it will shed some light for you.