This is the very same pane that comes up via the Page Setup process.You will see that this shows the Printer page size as Width = 420mm and Height = 297mm, which is what has been set up on the Page Setup window – i.e. A3, Landscape. Note that this is a full size A3 page with no printer margins.
Drawing size: You can set the size of your drawing in two ways:
Relative to the Printer Page Size. Rather than specifying an absolute size, you can make set your drawing size by a multiplier of the printer page size. Here we have set Width = *1 and Height = *1 – that is, the same size as the printer page size. You can use any multiplier you wish, using the asterisk as the multiplier sign.
Tip: I often set my initial drawings size to Width = *1.25, Height = *1 as this gives me a bit of extra space on the right hand side of the drawing for the Tool pallets to sit, without obscuring the objects on the drawing. Once the drawing is done, I set the width back to *1.
Absolute Size. Enter the actual size you want your drawing to be. So for a single A3 landscape page enter Width = 420 and Height = 297. This produces exactly the same result as entering Width = *1, Height = *1. You can enter any number you wish; for example, entering Width = 840, Height = 297, would produce a drawing two A3 pages wide and one A3 page high.
At best with margin: Checking this box overrides the Drawing Size entry and, instead sets a drawing size the specified number of pixels outside all the objects visible on the drawing. So at the default 25 pixel setting, the drawing size will be set to have a 25 pixels margin around the outermost drawn objects. Quite logically, objects on non-visible layers will not be included.