hi pip,
check your mac properties, if it is a PC powerchip, you may have to enable "Rosetta" in the indesign applications properties folder.
this may resolve the file print size conversion process.
Rosetta is inbuilt in mac with this type of powerchip, and when activated, works in the background..
http://www.apple.com/rosetta/ this link will give u an insight
not sure this is your answer....cheers rooboy