Although there are 32 mailboxes available in eCAN, for each mailbox you can send 64 bits of data.
Check out the "Data Vectors" section in the following documentation page for more information:
https://www.mathworks.com/help/supportpkg/texasinstrumentsc2000/ref/c28xecantransmit.html?s_tid=doc_ta#mw_61ad899b-f450-4b9d-ba4c-965499cc3470