Having had sailboats and pop-ups with and without sinks, I would consider the following when adding a sink:
If you are only using sinks as destinations for the water tank, install foot pumps instead of electric pumps to pump water. That way, when your battery is dead, you can still access your water. A foot pump works very well, and allows your hands to be free. I installed a foot pump in my first PUP and one of my sailboats when the cheapo hand pumps on the faucet broke.
However, manual pumps do not work in conjunction with hot water heaters. The hot water heaters require a pressurized water distribution created by an electric pump.
Water tank size is going to be dictated by space and weight concerns, although number of people times number of days should determine size. Water tank needs a vent, a tank drain, a fill pipe, and a draw pipe.
hope this helps