My map is 33.1 inches by about 15.5 inches. I'm assuming the original was printed onto A1-sized paper. (I'm pretty sure the Tally Ho was printed at A1 size as well.)
The ISO 216 standard for paper sizes like A4 and A3 etc, wasn't written until 1975,
As the Prisoner was made in the UK well before that, any paper used would have been imperial sizes.