“Target Size” refers to how similar the people using Quora in the Lookalike Audience will be relative to the source audience. A smaller number will result in a lookalike audience with fewer people in it but who are more similar to the source audience and may perform better. A larger number will result in a larger lookalike audience for more reach.
Select a low number if you want to optimize for the members of your lookalike audience to be as similar to your source audience as possible; select a high number if you want to optimize for higher volume and are okay with some members of your lookalike audience being not as similar to your source audience.
If you're unsure what to choose, you can start by creating a 5% target similarity and run a test campaign targeting that lookalike audience. If performance is good and you want to try getting more reach, you can then create a new lookalike audience at a higher number for testing. If you want to try getting higher performance, you can do the opposite.
Please note that the size of your Lookalike Audience is calculated when it is targeted in an ad set (under "Potential Weekly Impressions"). Lookalike Audience sizes are not calculated in the Audiences tab.