The size you select is governed by the maximum amount of power you need at one time. The most important thing to do is to assess what you are likely to power from the inverter and total the amount of watts they require. I would strongly recommend buying the 600W unit as there will always be times when you wished you 'd have bought a bigger unit. To find the power needed to run a particular appliance, simply refer to the specification plate on the appliance itself. That is the figure you use.
Some products have a startup or surge power required.
Often they are products with motors. For example, it may need 500W to run the product but need 700W to start it initially. In that case, the 700W is the figure to count. Batteries are specified normally in amp-hours. Hypothetically, you can estimate the battery life as follows using a the above example of 270W of products.