If you copy
WC3 folder on usb stick, make sure you delete all sreenshots, saves, all maps you don't need, movies and all that rubbish. Basically, all you need are .exe, .dll and .mpq files from main directory, everything else is optional. Unfortunatelly, that will still require 840MB
If that is too much, use some MPQ editor and delete data you won't use, like campaigns, from the archives.