retro would be the man to ask, but if I were doing it, and I refused to seek advice, I'd render at A5 x2, plus the length of the folding between the two, and render at 300 ppi, if not more, for a 300 dpi printout. as for A5 format... i dunno what the dimensions are, but it wouldn't take more than a minute or two to search up and find out.