On the 3560 and 3750 switches, you can tune the buffer allocation between queues with the:
mls qos queue-set output <qset-id> buffers <alloc1> <alloc2> <alloc3> <alloc4>
command. The alloc numbers are percentages and not absolute values.
I've searched high and low looking for the size of the (absolute) total buffer space on the 3560, but I have not been able to find it.
I would guess this buffer size is the output queue size of the interface, which you can tune with the hold-queue out <size> command? It defaults to 40 frames, if I'm not mistaken.
Anyway got experience with this?