This post is getting to long so I have split up the Paper size part.
Regarding paper size, since ISO is scalable, for most of the world, in many circumstances paper size can be irrelevant. The key point for ISO users (the whole planet, to at least some degree) is that when scaling do you, or do you not, want just the drawing scaled or the whole thing scaled (including all elements e.g title block). Page orientation and series (A or B etc.)
Often you want the entire drawing and all its elements scaled, normally I do, because you only print A4 because you don't have a larger printer, mostly, hence you want the title block scaled as well, so that you don't waste space on the smaller paper, you compromise by making all the text etc. smaller but if the text is correct on A3 it is still legible at A4, just smaller, all the entire world except those using ANSI have to do is just click "scale to fit" on the printer driver and your A3 comes out perfectly on an A4 printer, I have made many A3 drawings and then just printed them in A4, when my A3 printer died.
Luke, I was thinking what could be a really useful feature though is to print a drawing over two pages, from within FreeCAD, and intelligently define the joint margin.
For insertion into a "book" of A4 this works well sometimes. Also where a professional appearance is irrelevant, you can just put the two A4 sheets face to face then staple/sticky tape them together then unfold them. Of course with ISO sizes all being proportionate and scalable, this method applies to all sizes, so not matter how big your printer is...you could always print to the next size up.
Of course this scales beyond the next size, you could also possibly want print to four pages etc.. e.g. A4 to A2. etc.
You do have to deal with the issue of the joint margin, unless you have a border-less printer and some stick tape
I have in the past had a printer driver with this functionality and it is common the other way around (putting tow pages on one etc.).