If you can, I'd skip the inverter all together. Turbs post before was right that the appliance will have the same draw, no matter what it's plugged into, so the only thing you can do is mitigate the amount of loss that comes from doing too many conversions. To charge a cell phone, in most cases you can go:
12V DC -> AC -> 120V AC -> DC -> 5V DC (using an inverter)
12V DC -> 5V DC (using a cigarette lighter style USB adapter like this
which route would lead to more loss?
So, first look at what you actually need AC for. Then, whether or not you can replace those with AC less appliances. For example, somebody mentioned a shaver. I replaced my plug in shaver with battery powered one that runs on Double AAs (Like this
Then, if you still really, really need an inverter. In theory, a smaller inverter might be more efficient. Or it might not. Smaller often means cheaper often means more cutting corners... so it's also possible that the smaller inverter can cause a higher percentage of loss than the larger inverter, if it was more cheaply made.