checkbox list in metabox

Hello everybody. I’m struggled with this. I don’t know how to do. If someone could help me, I’ll thank forever and ever.
I’m workin on custom metaboxes with more than three checkbox lists in them. These lists are all dress sizes (men, women, children). My client in this way is able to select all the sizes available. A visitor would see just those selected.
After a couple of weeks of programming by myself, I’ve found this script
https://www.deluxeblogtips.com/meta-box-script-for-wordpress
that is exactly what I need. In the example provided, the author gives instructions about retrieving data from checkbox lists.
The instruction is
if(in_array(checkbox_list[‘…’] { do stuff }
where … is for the key of the wanted value
Apart from the typo, this instruction can’t give me the data I want, or actually I don’t know which php instruction I should use.
Anyone can help me? Please!!!
Thanks in advance