Inverters operate most efficiently when they are in the upper range of their capacity, but not so close to maxxed-out that they run very hot. That said, your inverter
needs to be large enough for the very largest load you put on it. That is to say, if your average load is 300 watts then you might think a 400 watt inverter is perfect, and it would be, until the day that you try to run something that needs 600 watts.
So, you want one that is big enough to run your largest loads -- with a little extra, just to be sure -- but not a whole lot bigger than that.