lv2 host buffersize

[gm-linuxaudio . privatize920]

Hi,
Im at a loss trying to understand how to access runtime parameters in
an lv2 plugin at init time. Specifically the buffer size.
I understand the concept of the LV2_URIDs and have mapped the resource
LV2_BUF_SIZE__maxBlockLength to its urid:

typedef struct {
        LV2_URID_Map *map;
        LV2_URID samplerate;
        LV2_URID buffersize;
} GuitarMidiURIDs;


static void map_options(LV2_URID_Map *map, GuitarMidiURIDs *urids)
{
        urids->map = map;
        urids->samplerate = map->map(map->handle,
LV2_CORE__sampleRate);
        urids->buffersize = map->map(map->handle,
        LV2_BUF_SIZE__maxBlockLength);
}


Now I cant find in the examples how to actually get the buffersize
given urids->buffersize. Can someone point me to a plugin where this is
achieved? 
Thanks,
Gerald