Verm: the units are whatever you want them to be. If it suits your game world, it's handy to consider them metres.
As you generally need to stay within -/+ 10000 units to avoid floating point accuracy 'issues', you might want to scale everything down in order to have a larger world and so you might consider the units, say, 10 metres... or kilometres/miles, or whatever. Elephants is a perfectly valid unit of measurement if you decide on a standard fixed size of elephant... :)
As you generally need to stay within -/+ 10000 units to avoid floating point accuracy 'issues', you might want to scale everything down in order to have a larger world and so you might consider the units, say, 10 metres... or kilometres/miles, or whatever. Elephants is a perfectly valid unit of measurement if you decide on a standard fixed size of elephant... :)