You could fill the pot up to around 4 gallons and start straight into it, the seedling will stretch a bit and as it grows you could fill in the rest. My autos have all been started in small pots and transplanted up. As long as you don't damage the roots then there shouldn't be an issue, but do what your comfortable with.
You could start in a larger pot but keep in mind the larger pots hold more water so it'd be easier to over water the seedling. I find its easier to water and care for them in a pot appropriate for their size, I think I'd kill a seedling in a huge pot lol
I don't have too many grows under my belt either, I just read a ton. If you're interested in organics there's recipes for soil mixes all over the place, and after the initial investment it's seemingly pretty smooth sailing...
I'm no expert, that's for sure. I just figured I could answer your question because I'm currently growing an auto in organic soil