In Europe and most other places, the size of a printed Pycon poster is A0 (1,189 mm × 841 mm) Paper size - Wikipedia
@encukou asked me what size was used in the USA and I didn’t know for sure.
Edit: apparently the USA uses Arch E/Arch 6 (48 x 36 inches) (thank you @ngoldbaum )
Since this is likely to come up sporadically, I thought it made sense to post here. I can edit this with the authoritative answer.