I run into this from time to time where more Senior and Staff Engineers take interesting projects. I’m usually left with ones that take medium time and medium impact. How do I find projects for myself that expand my impact?
I can't tell if you are looking for promotion into senior or staff, so I'll link this very recent question about staff level promo here. Rachel's discussion on staff engineers literally discusses how at Staff, you have to find your own projects.
In general though, I think this is fantastic question because figuring out what problems exists and how to solve them are inherently important to your growth as an engineer. While there are top-level directives that results in big projects with huge impacts, often projects start with smallish estimates and become bigger projects and bigger impact, because the engineers on them are looking out for the correct risks and ways to leave things better than when they started.
There's a couple ways I've personally seen people of all levels find projects, and one key insight is that often people weren't aware these projects.
Lastly, you want to prioritize these projects by leverage - the impact the project would have divided by the amount of effort it takes. Impact doesn't have be material or necessarily measurable (it could be trust earned with important members of organization, more trust in the product or people in the org, etc), but it's good for you to have an expectation for what would the impact be if you invested your time and energy.
Alex actually has a great talk about finding scope with an example from his time at Instagram: https://www.jointaro.com/lesson/WYk4J05iBqOuPtD0CYKX/session-1-solving-a-multi-million-dollardollar-instagram-bug/
On top of Kuan's excellent answer, I recommend going through these discussions too:
In general, finding a good project isn't too different from building a good product: Work backwards from problems. Be acutely aware of pain points (there will be a boatload in any engineering team), and from there you just need to evaluate solutions and stack-rank them based on the dot product of time required and impact.
You already have the first step in solving this problem which is realizing that you need to find these opportunities instead of waiting for your manager to hand them to you. The next step is to go out there and start exploring the problem space!