Root ball size?

Well, for a start do you have to move it now? The autumn would be a better time.
And prune it slightly first then excavate its roots slowly. Think roses are really quite tough main root might be deep you might have to cut it.
Just don't strain yourself.

As it’s quite straggly I would wait until autumn then prune it back quite hard, getting rid of all spindly growth.

Loopyloo is right about the main root so to avoid straining yourself get as much out together with side roots as you can then cut the plant free with loppers. Try and match the amount of pruned top growth to the amount of main root extracted - root to shoot ratio. This will prevent the plant wasting energy on parts it can’t sustain.

Prepare your new planting hole so it is wide enough to incorporate all side roots and deep enough to allow 1” below the graft point to avoid wind rock while it ‘finds its feet.’

I had to move a large climbing rose from the front of my house when I had a porch built on. It had some historic value as it had been grown from a cutting taken from the local 'big house' by a previous owner of my house.

I cut the rose down to practically nothing (I had to as there was no other way of moving it) and got out as much root as possible. I replanted it but also stuck about a dozen cuttings into a bit of spare ground. Not only did the original plant survive, but most of the cuttings rooted too. I was giving them away left right and centre!

Roses are remarkably tough.

Even if you find it too hard to get the original plant out I think it would be worthwhile to take some cuttings, some are sure to take.

I would agree about Roses being resilient.
I had a fairly new rose by my back gate and workman trampled it daily while erecting scaffolding next door
I asked them to take care but they were oblivious. I managed to dig it up and replant it in a safer spot and this year it has blossomed again.
I had help lifting it so do ask for help. People like helping

Thank you all. Yes, I can cut it well back and you’re right I should wait until autumn - hadn’t considered that. Thanks for the info about root to shoot ratio merlotgran, I’ll definitely bear that in mind - & btw I WANT that username!

A tip somebody told me for moving roses is that once you’ve given them a hard prune, down to about a foot tall for each stem, is to wrap round and round the plant with bubble wrap quite tightly. That way you can manoeuvre it while you’re digging it out without getting caught by the thorns.

Thanks, Casdon.

Apparently it’s important not to get scratched so that’s really useful

Also make sure you have some really good gardening gloves to withstand the thorns. I find rigger gloves very useful and they don’t cost a fortune. Also when you plant it on put some potash in the planting hole or a banana skin. Water it in well. Hopefully should be ok.

As suggested above, taking cuttings would be a good idea. Roses generally take quite easily.
Is there no one at all you could ask to move it for you when the time comes? No matter how careful I am, I always manage to get scratched, and as that’s a big risk for you, maybe a gardener might move it for you. I know you’ve said you don’t know of anyone, but maybe an online search might yield some results?

Once you have pruned it back well it will not be so daunting. I would add a sprinkle of fish blood and bone to the planting hole, spread the roots out as best you can, keep the graft union above the level of the soil. I would prune well again in March and top dress the soil around the rose with more blood fish and bone, with some well rotted horse muck as a mulch.

As others have said they are extremely resilient.

I think if you wear a thick jacket, jumper and thick gardening gloves you will be safe enough from thorns.