Indeed. It's to slow them down more than anything.

The issue with going for longer chains is that they're more likely to end up hanging down to the floor, which makes the job of cutting them easier.

All these fancy chains from Almax and the like will resist cropping but a fairly small, cheap battery angle grinder will still chop through them in no time (albeit with a bit more noise).

Yes and no is the answer to that.

Yes when the practicality and weight of such a big chain could damage what it is securing.

No as the bigger the chain the harder it is to cut.

I went for a pragmasis 13mm chain and was surprised by the size and weight of the thing when it arrived, it is a big chain, much bigger than anything you will see in a local diy or bike store. The other thing to consider is how the chain is made, Pragmasis and Almax i believe go through a specific heat treatment to make them as strong as possible without getting too brittle, you can end up with very hard to cut chains that will just shatter if smashed by a mallet if the heat treatment is done wrong.

Ultimately hoever, if they want through the chain and cant crop it with bolt cutters then a battery powered grinder will do the job.

My advice is get a chain that meets or exceeds the requirements of your insurance policy (solid secure gold for example) for your bike and is practical for use in that if it slips out of your hand when unlocking it is not gonna wreck your frame.

You're mixing up two completely different things. Lifting chains are designed for tensile stength, ie lifting stuff. Security chains are designed to be impervious to cutting (ie hard). These two charactreistics are mutually exclusive. Your lifting chain would be really good at resisting a thief grabbing both ends and trying to break it by pulling it apart, but will be slightly softer than a decent case hardened security chain. So the thief will be able to cut it relatively easily.

In response to the OP, be cautious about using a thin bit of string/rope to measure required length. The chain is hugely bulky and so needs a much longer cifcumference than a piece of flexible string. Buy much longer than you think you need. I stupidly took Steve's advice on length and bought two 2.5m chains. They were too short and really annoyed me. In the end I went back and bought a 4m one as well. It's much much better.

I have been looking at lifting and anchor chains as they seem to be cheaper and buyable by the metre. It is possible to get in 13mm (£15/m, 3.8kg/m), 16mm (£20/m 5.6kg/m) and even 20mm (£30/m. 9kg/m).

Most standard 3ft lever bolt cutters are advertised as being able to cut up to 12mm, which suggests that a bike secured with anything thicker than that would require a little more effort or forethought to steal. Although if somebody is really determined then they will find a way, or just strip the frame.

Whatever thickness I get will be wrapped in a tube to protect the frame, ground anchored and have a beefy padlock, but is the additional weight and expense of a 16mm or 20mm chain worth the hassle?

I bought 2m of 13mm lifting chain and slid it into an inner tube, seems to do the trick and cost about £40.

the sliding it into an inner tube takes a bit of time.
While looking for alternatives to big expensive chains I came across a thread where the guys were claiming that standing on 36" bolt cutters could just about cut through 6mm grade 80 lifting chain. The chain is easy to get off ebay and in various lengths.

I bought 2m of 13mm lifting chain and slid it into an inner tube, seems to do the trick and cost about £40.

Thanks, that what I thought I'd need to do