The optimal size? Changing the filesystem isn't going to gain you much, and I imagine throwing even a few 1-byte files on that drive would lose you all that space.
Look at how this goes: your 700 MB file is actually, let's say, 726,847,400 bytes long. That's 177,452.9 4k sectors long, and 88 bytes wasted. If it were instead 32k sectors, it's broken into 22,181.6 chunks, and 20,392 bytes are wasted. So in this case 32k block size hurts you. There's a reason 4k is the default - it works for a *lot* of things.
Granted, there's overhead I'm ignoring; this is a very simplified view of a filesystem. But you won't gain more than a few megabytes by using a non-standard block size, and non-standard configurations are harder to work with when you have to recover from them, back them up, etc; third-party utilities may assume that you're using 4k blocks and fail to perform on a differently-chunked filesystem.

