The statistics tab of your SpaceFarmer dashboard allows you to monitor three important things:
Your farm size is calculated based on the points you deliver to the pool over a 1, 7 and 30 day interval. Note that this represents an effective farm size, meaning that if you produce late or invalid partials, those will not count towards your farm size (see below). The variance in points is explained by the quality of your partials and the difficulty level you set.
Firsts of all, the pool doesn't know your exact farm size nor the amount of plots you have. The size is calculated from the amount of points you delivered to the pool. The points are like blocks. You don't know when they'll come, but you can do an estimation. The pool sums up your points from the past few hours and by knowing a k32 plots delivers 10 points every 24 hours on average, the pool estimates your farm size. During a lucky period, your size can be more then you have, but it can also be lower during unlucky periods.
When your farm size is really off by a big amount, please check the following: