OK, i worked it out. it is rather convoluted in the OSX print dialog. in order to change the page set up, one must go to Paper Handling > then check 'scale to fit paper size' > then change destination paper size (and feed).
When saving print settings in PS, one should bear in mind the paper size and feed in Paper Handling to be selected accordingly otherwise the page setup will be generic and unspecified. Of course, this is a little immaterial as usually the page setup is done prior to photshop printing and this issue is only a problem if printing outside of PS.
