We have recently implemented a feature to gather an estimated size from farmers.
You can view that information in the farmer details, within the Explorer.
Note that this information is merely an estimation/approximation of the farmer plots size. It is calculated through the partials that are sent to the pool. This means the value may fluctuate and not be constant, even if you are not plotting any new files.
DISCLAIMER: It will take about 24 hours before that estimation can reflect the farm size.