64-bit should be fine (unless you have a very particular reason for suspecting that)—that DLL should work on 32-bit and 64-bit architectures. You're running a more recent version of Windows than the library was compiled on, though, and I do wonder if something changed in an update that's causing problems. I would be kind of surprised, since it's very a simple library and Microsoft is pretty attentive to backwards compatibility, but if you're willing to build the library on your end it would be great to rule that possibility out.
The easiest way would probably be to install Visual Studio Community, then download this source archive and extract it wherever you like. If you then open the extracted GetFileSize directory and double-click on `GetFileSize.sln`, it should open in Visual Studio. From the menu bar, select Build -> Rebuild Solution. You should then be able to find the file `GetFileSize.dll` in the `Release` directory within `GetFileSize`. Replace `SongOfHomunculus/lib/GetFileSize.dll` with this new file, then try running ~Song of Homunculus~ and see if anything changes.
I can also walk you through using mingw if you would prefer not to install Visual Studio. If this seems like too much work, also, let me know and we can try a different tack.