I dont know of a way to change the buffer size but if your connection is good enough then its not needed. Remote playback works and even transcoded streams work if the internet connect has enough upstream on the serving side.
I have been playing around and using this rendezvous proxying thing a lot and had no issues at all with a 4/1 MBit connect on one side and a 16/2 Mbit on the other side.
But I am not using any VPN, just both networks are permanently on the internet with a flatrate and the appropriate ports open (with a password set in mt-daapd).
So the rebuffering SHOULD not come from having too low buffers. The buffers are ok for a normal internet connection.