It's a little tricky to pick label and feature styles (line thickness, colours, etc) in the map canvas because they don't reflect what I would see in my print composer which is at a different scale because of paper size and such.
I haven't found any options in the QGIS options or project properties.
Best Answer
In QGIS it's not called Reference Scale.
You can make sure line thickness and font size are visually the same in the map canvas and the print composer (regardless of paper size and extent) if you size your font labels and line styles using Map Units instead of points, mm, etc...
There is also the option to use non-map units, and in that case you can just set the map canvas scale to match whatever scale you want to print at to get a quick visual (but not if your map takes a while to render).