It all depends on what you are wanting to run.
I will not use anything smaller than a 2500 watt in a service van, but then again, I do have a habit of running a mig welder off mine
If you are only going to power up 1 machine I would not go with anything smaller than 800 watts and that is pushing the envelope.
Run the thickest and shortest cables you can afford to your battery for the best performance.