The Media menu allows you to select different paper sizes; if the paper size can be determined from the PostScript file, then that paper size will be selected by default, otherwise, the value of the defaultPageMedia resource (see Application Resources) will be used.
mgv understands the standard media from Adobe's PPD (sizes are listed in points): 10x14 (720x1008), A3 (842x1190), A4 (595x842), A5 (420x595), B4 (729x1032), B5 (516x729), Executive (540x720), Folio (612x936), Ledger (1224x792), Legal (612x1008), Letter (612x792), Quarto (610x780), Statement (396x612), and Tabloid (792x1224) Letter is the standard North American paper size, A4 is the standard paper size in Europe (and probably the rest of the world). One inch is approximately 72 points, one centimeter is approximately 28.3 points. Letter and Letter Small are the same size, A4 and A4 Small are the same size.