It is very obvious thing to determine the size of lists/libraries especially when you need to migrate/move the lists/libraries/documents across the site collections and choose the correct approach form migrating lists/libraries.
To know the size/content of Libraries/lists data, go to site settings – Storage Metrics under Site Collection Administration section. This page will display the metric info in-terms of name, size, % of size quota used, Last modified etc.
You can even dig inside deeper with the sites,sub sites,Lists, Libraries etc to know inner level metrics as well.
You can even go directly to this URL:
https://<YourSiteURL>/_layouts/15/storman.aspx
My Sharepoint Resource 