Incoming and outgoing CAN messages can be buffered in three different places:

In the CAN controller
On the CAN board
In the CAN driver (i.e. CANLIB)

The sizes of the first two buffers are clearly hardware dependent. You might find the following information useful.

Many CAN boards from KVASER uses the SJA1000 CAN controller. This chip can buffer somewhere between 5 and 19 incoming messages. It does not buffer outgoing messages.

CAN boards with an on-board microcontroller (LAPcan, LAPcan II, USBcan) typically buffer something like 500 incoming and outgoing messages.

CAN boards using the M16C microcontroller (USBcan Rugged, USBcan II, PCIcan II) typically buffer some 100 incoming messages and around 50 outgoing messages.

CANLIB drivers will, by default, buffer 256 outgoing messages per channel and 1024 incoming messages per channel. You can use canIoCtl() to adjust the size of the receive buffer.

Message Queue and Buffer Sizes