Once you know the worst-case current your Pis will require during operation, which will probably be when first turned on, consult a wire ampacity table to find the lightest useful gauge for your run, then use the resistance per meter of your chosen gauge to sketch out your run, including the appropriate wire resistance between nodes to detemine voltage drop between nodes.
Are you sure there isn't a more appropriate, less power-hungry platform for what you want to do? I'd think ESP8266 should be plenty for light-duty control and data submission over wifi.