The script gives you details about the updates in an update group including the size of each update. Mind that a download for a certain kb article may consist of different installation files for different releases (like x86, x64, amd etc.) or different languages. The report should help you to figure out how much content will be downloaded to your machines in the field. The output is an html report.
Download options:
From here: GetUpdateDetails
From GitHub: https://github.com/Maxwitat/GetCMUpdateGroupDetails
Example of the output
Links:
CM User Voice https://configurationmanager.uservoice.com/forums/300492-ideas/suggestions/18328069-show-patch-software-update-size