Length of high voltage power transmission in different countries?
The best page for the European system is probably: 
https://www.entsoe.eu/data/data-portal/Pages/default.aspx

As an example, look at NORDEL: 
https://www.entsoe.eu/fileadmin/user_upload/_library/publications/nordic/annualstatistics/Annual%20Statistics%202008.pdf

There you see the kind of numbers you ask for. 

Also, Google "entsoe order grid map" (I can't post more than two links). There you can get a detailed 26MB map of the european grid.