Do not know the size -- cannot find one listed in my 73 parts catalog, either.
Assume it is a standard size for the era -- 6"x9", maybe 6"x10".
I am sure IH sourced radios / speakers from a "standard supplier" - like ac-delco.
I would measure the distance between the 4 mounting "holes" (speed nuts) to get an idea of the size you need.
Then, I would look on the web for companies that supply restoration parts for 60's GM cars / trucks. I am sure they must supply new speakers -- if not 100% exact copies, it would at least fit.
Another (more expensive and time consuming) option is to find one in a junkyard vehicle and have it "re-coned".
