6 Volt question

[Kpar]

Question for members who are more knowledgeable than me regarding electrical. I do know that the bigger the size of the cable the less voltage drop you will get. If I run a 6 meter cable from the battery what size do I need to keep 6 volt at the end. Is this only relevant on what current is being drawn from that cable ?. Reason I ask this is I'm using fog lights for indicators through a couple of relays what size wire for power to the relays. So far I have destroyed 2 relays ?
Comments much appreciated
Keith