print-composerqgis
How do I fit the Frame (A) of the Map Item exactly to the extent of the Map Canvas (B)?
I have done all of the following:
These together were insufficient to move A to B.
Well, With Qgis 2.0, you can click the "Layout" menu, then "Move Content", and adjust the layers to your liking
The scale will inevitably effect the extent for any given area and that will be limited by the size of your paper (so an A4 sheet @ 1:50,000 scale will be approximately a half of the possible extent of an A3 sheet). Contrariwise, setting the map extent for a given area of paper will also inevitable dictate the scale. You cannot adjust one without influencing the other (which is why you have received some negative scores for this question).
You have stated that your actual extent is not as important as scale. So to resolve your dilemma, set the scale and then, with the map selected, click the 'Move item content' button (icon is a small arrow and green box, next to the 'Select/Move item' button - a large arrow).
You can now 'slide' the content of the map without changing the position of the map on the page and thereby center you map contents on your area of interest.
If the extent of the map is still not large enough at the scale you must use, then you need to increase the size of the map and that might mean splitting your map over two or more sheets and to do that effectively you can use the Atlas generator.
Best Answer
Firstly, it seems like the extent of your data and your map in the composer are of different ratios - if you want your data to completely fill up the map item in the composer, you will either end up cutting out some data, or there will be some white space. So you might want to make sure your map item in the composer is more square than rectangular.
You could do this by clicking and dragging the corners of the map item using the Select/Move item tool (third down on the left).
Alternatively, you could set the extents of the map item to the extents of the largest item in your canvas (the raster background, perhaps - right click -> Properties -> Information -> Extent). You could also set the map item extents to a vector layer in your project by using a data defined override with x_min($geometryoflayer) or similar functions. I'm not sure how to do that with raster layers, I think you need custom Python code.
Otherwise I would try changing the scale of the map in the map item and moving around the content until you are satisfied, using the "Move item content" tool on the left hand side (like a map with four blue arrows on it, 4th item from the top). You can click and drag, or use the mouse wheel to increase/decrease scale. For fine tuned scale adjustments go to the Item Properties for the map and try out different scales. Note that your map item needs to be unlocked in the print composer for this to work.