water heater usage

I'm opening up a new coffee shop and working with an architect on the buildout. I need to get an idea of the size water heater I need from other shops similiar to mine. My shop is 2000 sq. ft., seating for about 50-60 max. I'm only serving coffee, smoothies and pastries, no cooking. I'll only have cups, saucers, and eating utensils to wash in an undercounter dishwasher and/or 3-compartment sink. The landlord currently has a 10-gallon water heater but I know that's not going to be large enough. Anyone have any suggestions?