Le portail francophone de la géomatique

Page size is set in points (1 inch = 72 points), so if you set the page
size to "1191 842" you get a landscape A3. Then with the "Location of
map on page" parameter you define an area on the page (2 pairs of page
coordinates, in points, measuring from the lower left corner) in which
the map is placed and by setting the "Map Extents" you define the
geographical area (in map units) that's to be shown inside the map area
you've defined. Note that these last two parameters are optional and if
the length/width ratios don't match up the writer will compensate for that.

The help for the Geospatial PDF writer is pretty informative, all of
this can be found there.