This isn't really an answer to your question, but I'd pick one of those large codebases and read through it until you understand everything that's going on in it. Not only will your coding skills increase to the point that you're more confident and not completely intimidated by the other contributors, but multiple places to contribute will become clear.
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For someone just getting started, I'd be more inclined to recommend picking a smaller codebase.
Makes sense. For me it was important to pick something that I regularly used. Picking a smaller project will make it easier to mentally decompose, but you'll also get a leg up on a larger one if you're really familiar with the endpoints. Of course, you can do both:)