Read long characteristic response limited to 512 bytes

Hello,

Got a question regarding the response on a read long characteristic.
It seams that requesting a variable through a read long command on the gatt client side will always result in a data send by the gatt server (dialog) of 512 bytes (in chunks of 20 bytes of-course).

Here some code snippets:

#define SIZE_OF_DATA_BLOCK 4096 /* This compiles fine, but the gatt server will only send the first 512 bytes */

uint8_t DataDumpVar[ SIZE_OF_DATA_BLOCK ];


ble_uuid_from_string( "SOME-UUID-ID", &uuid);
ble_gatts_add_characteristic(&uuid, GATT_PROP_READ, ATT_PERM_READ,
SIZE_OF_DATA_BLOCK, GATTS_FLAG_CHAR_READ_REQ, NULL, &custs->DataDump_val_h);


static void handle_read_req(ble_service_t *svc, const ble_evt_gatts_read_req_t *evt)
{
svc cust_service_t * cust = (cust_service_t *);
if(evt->handle == cust->DataDump_val_h){
ble_gatts_read_cfm(evt->conn_idx, evt->handle, ATT_ERROR_OK, SIZE_OF_DATA_BLOCK, &DataDumpVar[0]);
}
}

--> this code has been simplified to support the question <--

The code above works, no error, no disconnect, just a limit of 512 bytes get send.
EDIT: sorry just checked.SIZE_OF_DATA_BLOCK 4096does compile but does NOT run on the device.SIZE_OF_DATA_BLOCK 512works fine

Now my questions is twofold:
1. where is this 512 bytes limit defined?
2. some thought has gone into setting this limit, question is, why 512? And what are the dragons I'll face if I increase this limit, preferably to 4096, but I guess the bigger the limit the bigger the dragons.

Thanks,
Robert