How is this sentence written correctly in maths? (sets)

A subfamily of $l$ sets $A_i$ with $1 leq l < n$ is a "critical family", if its union contains exactly $l$ elements.

(Information so you know what these symbols mean: $X$ is a finite set and $A_1,A_2,..,A_n$ are subsets of $X$)

I would write it like that $$left|bigcup_{i=1}^{l} A_iright|=l$$

I hope you can tell me because I’m using this definition in a presentation and I don’t know if it’s written correctly like that?

I’m not sure if I really need to use these cardinality symbols ||