the important speed is the upload speed for this ... if you have slow connection, use small buffer, if fast, use big buffer ...
there is setting for "modem", "ISDN" etc. ... that should help
if your upload speed is imax 550Kbps, it is about 70KB/s, so the buffer should be about 32kB, you can try 64kB too, but I think it would be good only when you upload one file parallelly