Remember that very small audiences can get exhausted quickly, which eliminates their ability to scale.
As a general rule of thumb, you're recommended to create your lookalikes from "top of the funnel" custom audiences. So prioritize events like purchases over "add to cards", leads over clicks, etc.
For the best possible LAL, make sure you have at least 3000 people inside your custom audience using your Facebook pixel. And pay attention to % thresholds. Start with 1%, then 3%-1%, 5%-3%-1%, etc.
If you want to stay at the top, you need to keep expanding. But be aware of 'ad fatigue'. Ad fatigue means, simply, that the more you spend on your campaign and extend your reach among current audiences, the more the frequency of your ads keeps growing. As a general rule, if your frequency goes above 3, your CPA starts rising dramatically, and you need to replace your campaign (or change the creative elements of your ad).