Very nice work. Glad that OSPi inspired you to design your own board. A few quick comments:
– The COM (common) port just needs one pin. The reason I have two (which are connected together) is to re-use the 2p terminal block so to reduce the total number of different parts. You can get rid of one to save some space.
– Regarding PCF8591, you can easily replace it with an ADC chip of much smaller package, such as MCP3021, which provides one ADC channel and takes almost no space.
– If you want to add a rain sensor port with minimal impact to the board size, you may want to take a look at the microcontroller-based OpenSprinkler:
http://rayshobby.net/wordpress/wp-content/uploads/2013/06/os20_component_diagram-1024×599.jpg
where the rain sensor port is the blue-colors screw terminal located in the interior of the PCB.
