Computer shutdown while downloading mojave, and now there is not enough space to download
First empty your trash (just in case). Then open Finder and hit Command + F. You should now get an empty Finder window with the option to add search parameters:



The first one, Kind is Any is already there for you. At the right side of this row click the "+" icon to add a parameter. You can see the ones I chose are:


File Size is greater than 4 GB
System files are included
Content modified is today


You may have to go into the "other" section of the dropdown menu to find "System files". In sum, this Finder search locates all files, including system files, that are greater than four gigabytes in size and modified today. Tweak the parameters if you don't believe you've found what you are looking for, and carefully review any matches so you don't delete crucial system files.