this is the internal buffer used by the fx engine, so for delay lines etc.. (as far as i remember)
the optimum size would need to be calculated.
no you cant just set it to whatever you like -as if its set to low, then it will just starting writing over other data you have in sdram... which would cause pretty odd errors, since you have no idea what other data it will be corrupting.
so no dont change it
when I have time, I'll look at trying to determine what it needs.