First check with printers about the resolution for a print of that size. max, 200 DPI is too much. Rosi might get by with something as low as 25-50 DPI. Or with normal screen resolution of 72 DPI. Big billboards are printed at way low resolution, since they are to be viewed from much further away than a 50x70 cm poster, let's say. I printed such posters at 125-150 dpi and they looked completely satisfying. So you could end with a render that's 900 x 6000 or even 450x3000. That's not so terrible to render.

-- erlik