In their paper on incast, Yanpei Chen, Rean Griffith, Junda Liu, Randy H. Katz and Anthony D. Joseph said:
To the best of our knowledge, the Nortel 5500 series switch has a maximum of 131072B of buffer space for each port, and 786432B total buffer space shared between 48 ports.
It is interesting that serious academic scholars need to hedge on a detail that is central to the performance they are studying. It is testimony to the difficulty of getting this information out of manufacturers.

With Nortel's backruptcy, the ownership of the switch line was purchased by Avaya. The Nortel 5300 family appears to have 12-port shared memory switch modules connected with some sort of central switch. Early software divided the packet memory into eight queues. Starting with version 4.0, command line controls were added that permitted reduction of the number of QoS levels and hence queues. This permitted fine tuning of the packet memory distribution to match requirements. At the time of the incast paper, the number of queues could not be reduced and so the max burst absorbtion was 131 KB. Subsequent software permitted this to be 786 KB.