How does one know the margin size of a given printer? I have designed maps in composer, changing size and position of map frame by trial and error. Eventually I arrive at the biggest map I can produce on A4 that is not cropped, but if I then export to PDF then attempt to print that I either get the map drawn at a different scale if I 'fit to page', or the edges cropped if I don't. Is there a similar feature to ArcGIS where the print extent for a given printer can be displayed in the composer?
Until this feature is introduced in QGIS, here's a workaround.
Open a Microsoft Word document. Make sure the printer in question is selected. Set the page margins to zero and click OK. You'll get a warning telling you that the margins are too small; click Fix. The print margins will be reset to the smallest possible margin that the current printer will accept. Now you know how much space to leave around your map in the print composer.