In general, the hot water flow rate of a small tankless water heater can support a bathroom and a kitchen with a dishwasher. The key to selecting the proper model is knowing the flow rates of your fixtures, the coldest temperature of incoming water, and your usage patterns (how many faucets do you expect to be able to use at the same time?). Work with your plumber to choose a size that fits your needs.
You can also install more than one heater to heat water by zone. For example, at my current remodel I’ve installed two tankless water heaters—one supports a kitchen, bathroom and half bath; the second supports two additional bathrooms.