labelingqgis
Is there a way to set the labels size of font to fit the scale of the map in the composer window but also to fit in the main window?
The map in the composer is size A0 and the scale is 1:5000.
In order to see the labels clearly in the composer, I set the labels font size to 23. So in the main window it's too large but in the composer it's OK.
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.
You can make the font size constant in metres rather than points, so it will scale with the map. In Layer Styling, select Metres at Scale instead of Points.
You can also make the size in points (or metres) a function of polygon area, using an expression.
The polygons below are 10000 square km on the outside, and smaller in the middle of the map. The expression for the size (metres at scale) was simple:
$area/200000
A more complex formula may be useful.
Zoomed in, it scales with the map:
You can base the font size on the area of the polygon.
Best Answer
In the labels properties choose a data defined properties for the size (drop-down menu on the right of the field) and plug-in this expression.
You'll just have to replace 'mymapincomposer' by the name of the map item in your composition. Just adapt the expression if you have multiple composers. please note the id name for the main windows is 'canvas'.
Enjoy!