Controlling line width in QGIS Print Composer output

Question

I am printing a 36" x 48" map (ARCH E size) and looking to have line width consistency for my contour lines. I want to be able to define the line width as it is printed on a 36" x 48" paper at 300 DPI.

What do the absolute values of mm or pixels refer to when describing line width? Will they stay consistent on an exported PDF despite the map's scale or size? Can I use these absolute values of mm or pixels or must I use map units?

I have a small reference map with a colorized elevation gradient as a reference, and would like these line widths to print the same as the line widths on the big map, despite their difference in scale. I believe I may be able to use map units, and as a function of the maps scale & size I could define a particular 'as printed' line width for any size map.

Attached is an image of the map I am working on.

Answer 1

Yes, sizes in mm will be constant regardless of the map scale or page size (note thematic they WON'T be constant if you resize the generated pdf, e.g. printing an A4 pdf to A3 size).

Sizes in map units will vary depending on the map scale.

Sizes in pixels will keep a constant size in pixels regardless of map scale. But the actual rendered size will be dependent on the output dpi - so a 1px line width will look bigger for a 96 dpi output vs a 300 dpi output.