Problems printing posters seem to be one of the typical stresses of being in academia.
I once developed my own poster entirely from scratch, including setting the page size on my computer exactly (using mms). Then, when I went to print it, I was told it wasn't A0, it was too big! I explained that I had set the page size precisely using mms so it *had* to be correct. The pdf info on my compute was correct. But when you opened the pdf on another computer in miraculously got bigger.
At which point the printer told me that mm on pdfs for Macs and PCs are sometimes treated differently. WTF?!?! Here I was thinking a mm was a standard unit of measurement. But apparently not. (Actually, I don't believe this guy but whatever).
Anyhow, long story short, I just printed the poster to be an two inches too wide. Took it to the conference. And it didn't matter in the slightest.