I have been playing about with an arduino ethernet shield, powered by a w5100, I was having a problem with some data being cut off.
After some digging around in the code, I managed to change the number of max connections and therefore allow me to increase the max buffer size.
This repo are the modifications I made, useless to anybody apart from me of course :D
For anyone that stumbles upon this place and is wanting to change the buffer size, here is what I found.
Enabling ETHERNET_LARGE_BUFFERS does not seem to fix the problem, in order to allow changes and basically have an unlimited buffer, you will also need to define the MAX_SOCK_NUM and bypass whatever is done with RAMEND and RAMSTART.
This is most likley the wrong way to do things, but it worked for me. See line 37 to 55 in Ethernet.h to see what I did.