I'd suggest going by what you use. At least for me, the initial motivation worked way better when fixing a bug on or improving something I regularly use. That can range from some random browser extension to tooling of a programming language you use. Sending PRs to one-person projects can be a bit hit-and-miss if they are active, but on the other hand they're often happy about any contribution they get that shows someone else cares about their thing. On the other hand, large projects are more likely to have a community where someone might help a first-time contributor through the steps.