Yes the precise length could be infinite…
That RV did it by following the GPS map and twiddling his heading bug, so no automation.
Also doesn’t the border pass through the Matterhorn, so you would need oxygen to fly that bit? I don’t think he actually did that.
He also states at one point “Approaching Geneva, stealth mode is the applied flying tactic.” which sounds like turning off his transponder 
To do this in the UK you just take off, fly, get any CAS clearances, and land. No flight plan needed. There are many DAs which would need prior coordination to cross and may have to be done on a weekend (but even then you would need to call. France would be similar and possibly harder unless you know the right numbers.
For an island country, or a country with a piece of coastline, would you fly the coastline, or the 12nm territory limit line?
Peter wrote:
Also doesn’t the border pass through the Matterhorn, so you would need oxygen to fly that bit? I don’t think he actually did that.
You can see in the video he climbs up to 15000 feet.
Peter wrote:
He also states at one point “Approaching Geneva, stealth mode is the applied flying tactic.” which sounds like turning off his transponder
If he would turn off his transponder, how did he show up on FR24? 
