Standardized scale for coastline length or mapping in general? Measuring the length of a coastline depends on the scale of the measuring tool. The smaller the scale of your tool, the longer will your measured coastline be. The reason for this is the fractal property of coastlines. This is well known since a paper by Benoît Mandelbrot "How Long Is the Coast of Britain? Statistical Self-Similarity and Fractional Dimension".

Now I was wondering if there is a standardized mapping scale to come up with global or country wise numbers of coastline length?

And furthermore, if there even is a general rule for all mapping purposes, which scale is to be used in mapping?

Is there maybe even something like an ISO standard for this?
One standard that has be used by the USA is to measure the distant between points on the coast at intervals of 30 latitude minutes, as measured on a 1:1,200,000 scale map.  

See chapter 5 of Measurements from Maps: Principles and Methods of Cartometry by D. H. Maling for further information and other standards that have been used.