The rationale behind this is that larger PRs are somethings ignored, as they take time to review. Smaller PRs are better, and are generally easier to review. My theory is that if my team members can see right within Slack that a PR is small, they might be more likely to review it.
this serves two purposes: as a reviewer you can see immediately which reviews are small, and therefore hopefully quick (XS is likely to be something like incrementing a version number or fixing a typo) so if you have limited time you can review the small PRs that are on your plate and save big ones for later and then, relatedly, for PR authors it gives them an incentive to break their work into smaller PRs when possible because they know it's likely to get reviewed quicker. this is also a good thing to do because if you need to roll something back, you can roll back a smaller chunk of work. and it's easier to catch bugs during review if you're reviewing smaller chunks of code at once. so anyway my suggestion is to adding sizing indicators like that. maybe even use some kind of emojis that represent smallness and bigness!
Requested by a number of folks: