How do I find the scale of maps?

[Gidde]

Interrupted just curves a little less. GProjector actually has a preset for interrupting based on earth's continents, so if you throw the blue marble in there and choose that preset you'll see.

If you're doing a lot of effects and not much drawing, the curve is great, but for hand-drawing (like I do) it's extremely difficult to curve it correctly. So the interruptions help me.

[Hai-Etlik]

All projections distort distances. Distances on a sphere are fundamentally different from distances on a flat plane. You can not have a consistent linear scale for a global map.

For an interrupted map, you can, potentially, keep the distance distortion reasonably small within each section, but you can not reliably measure straight line distances across discontinuities and to get the distortion low, you need a lot of discontinuities. At that point you are better off just making separate larger scale maps as you'll be able to select better projections for each if you don't need to line them up along shared edges.

As a rule of thumb, global maps should be in compromise projections (Winkel Tripel, Robinson, Aitoff, Waterman/Cahill/Keyes, Dymaxion, etc) for general reference such as an overview map in an atlas, and equal area projections (Mollweide, Hammer) for thematic maps of data related to area like densities. Other projections have specialized uses like Mercator being particularly suitable for "zoomable" maps in situations were dynamic re-projection isn't practical and plate carrée is good as an intermediate to be translated into other projections.