How can I copy files and only overwrite if filesize differs?

What I'm looking for is something that seamlessly works within the windows GUI, not a command-line utility like robocopy. I do lots of small copy/pastes of various files and I'd like windows to just always overwrite if filesize differs, and if filesize is identical then it does nothing. Can it be done?

You can use TeraCopy if you want, then just set up the settings. Alternatively, it also asks you what to do when file name conflicts exist during a file copy operation (so you can address it on a case-by-case basis). Also, if a conflict is detected, you can choose what to do on a file-by-file basis, or for the entire transfer queue.

TeraCopy also integrates with the Windows shell to replace the default copy-and-paste operation with it's own interface.

NOTE: As of September 2014, TeraCopy only works within Windows or with Android devices (smart phones and Tablets) that have Mass Storage Mode (Android Gingerbread and early versions of Ice Cream Sandwich). Newer devices that use Android v4.2 Jelly Bean or later have MTP mode only for file transfers and TeraCopy can not "see" devices that use MTP mode for file transfers.

rsync can also be used on windows, and it's a bit more flexible tool - it has command line options to compare timestamps only, but it also provides spme speedup in bandwidth constrained operations - like copying over a network or to a slow (writespeed) drive, since it's able to limit itself to transferring the parts that changed.