How do you determine the impact of projects?

how do I evaluate the business impact a project has and prioritize all the different things on my plate accordingly

In a small startup, this should be quite easy. I used the following filters

• can the business function without the success of a project
• what's customer visible impact if we finish this
• how fast do i need to finish this and what difference does it make

how do I make sure that the scope of projects are matching my level

You are in a seed stage startup. You should leverage that. Go to leadership and be upfront. You can directly ask them whether you are driving useful projects for the business.

source: I was an early employee at a small startup

Adding to Seed's great answer, I also wanted to call out that some of this depends on the company size. Since you're at a seed stage startup, it's probably too early/small to have formal levels.

In the early days of a startup, being senior effectively boils down to "people trust my opinion + feedback, and the work I'm doing is critical to the #1 company priority". It's going to be based more on gut feeling and perception rather than a well-defined engineering ladder.

Since you probably don't have a formal engineering ladder, it's interesting how you frame the question around what "exceeding" expectations looks like at your level. I would not worry about that. If you're senior at a startup, your goals and the company's goals to be closely aligned: have you found product market fit or not?.

(If your startup is much bigger, please do correct me!)

That being said, how do I evaluate the business impact a project has...

• For early startups, it should be pretty obvious why a project is needed for the business to advance, similar to what Seed mentioned.
• If you're working at more mature company, metrics are the way to go. Try to match to your team's OKRs/KPIs.
• This is more non-objective, but if an executive (i.e. a director+ level person) says it's important, it is important as they'll have a hand in evaluating your performance.

...prioritize all the different things on my plate accordingly?

• Weigh by the cross product of business impact and time spent. Get low-hanging fruit (low time spent, high impact) and after that, it should be a mix of high time-spent/high impact and low time-spent/low to medium impact.
• Talk to your manager regularly and make sure you're aligned on how you're prioritizing/splitting your focus.

On a side note, how do I make sure that the scope of projects are matching my level or exceeding it?

For this, I recommend this in-depth advice: "How to identify projects that are more suitable for senior engineers?"

And of course, talk to your manager (as always).