Size does matter when it comes to power stations. Depending on what you would like to power will ultimately determine the size needed. If you are just wanting to charge up your laptop and run a fan or lamp, then you'd be fine with a 300 - 600W unit. But to power something like a space heater or air conditioner, you'll want to look for a larger unit that offers at least 2000W.
Then you need to consider capacity. The rule of thumb for a few years was the output wattage usually matched the capacity. For example, a 300W unit would have around 300Wh of battery capacity. Now, with new tech and battery chemistry, you can get a power station that offers 600W of output from a 300Wh battery. Capacity and output still correlate because if you have 1000W of output from a 1000Wh battery, you can use the power station for one hour before it is drained if you are pulling 1000W. So, you'll need to get an idea of the devices you want to run from a power station and how long you'll need them to operate.