Counting of an Array within DB

Hi all,

Im trying to get the count of likes for each post when querying all posts.

I tried going through output > aggregate but it doesnt offer the option for arrays (i.e. posts.liked_by)
[1.PNG][agg.jpg]
I also tried going through eval but it seems to throw an error saying 'aggregate needs to be processed first'
[eval.JPG][eval fail.JPG]
Can anyone please advise me on how I could get the count of an array? Many thanks!