....the best comprise of weight verses usable length? They are available it 0.7, 1.5, 2.0 & 2.5 meter lengths.
Can you not work this out for yourself?
Take a piece of string 2.5m long. Tie knots in it at 0.7, 1.5 and 2m points. Offer the string up to your awesome steed and imagine that you are:
1. Having to tie the bike to itself, for instance just looping it through a wheel. Some might say useless by way of security, it being too easy to lift the bike up and carry it away.
2. Looping it through the bike, around something solid (a lamp post, say) and back again to the shackle. What's the minimum length YOU imagine you'll need to do this, having in mind that the solid object might - to some degree or another - be at varying distances from your bike?
Which length feels like it will suit YOU best, remembering that the string is a bit more flexible than a heavy chain?
Doing this will affirm your position as a self-reliant Adventure motorcyclist of the first order, able to buy a chain (weigh them up by hand in a shop, using imagination) without help. Only start another thread if you are really unsure how to carry the length of chain whilst out hooning.
PS The true Adventure motorcyclist will of course have chains of all lengths, ready for use as circumstances dictate. Popping out to Tesco: 0.7 Badlands of North Sudan 2.5m.... And a guard.