Hmm - 12mm Spectra has proved to be about right for a 62' masthead, inline rig on a 6.8 tonne racer-cruiser with a deep draft
bulb, three spreaders, permanent inner forestay from top spreader level, no babystay. Mast
fore-and-aft dimension 200mm or 8".
I think I posted a photo
a while back of it carrying a decent sized staysail and nothing else, with both runners set up, in 60 knots true, with no hint of a wobble or wriggle.
So that tallies more with what Dockhead wrote, by way of sizing.
Even when there's no staysail, the runners play a crucial role in keeping the mast
'in column' with in-line stays. I'm thinking in the event of severe loading, such as smacking the mast into the briny, or a flick roll.
And (again I think it was Dockhead) I entirely agree it's all about freedom from stretch: the angles are so favourable that strength is not the factor governing runner choice. A runner fails to do the job not by breaking (unless it's damaged), but by allowing the mast to move enough that the compression
load will buckle it.
It's like a heavy bloke standing on a soft-drink can when it's empty, and a bit dented, as opposed to unblemished.
I would argue that differences in the particular layout of the rig, especially such things as the fore-and-aft mast dimension and whether/how much the spreaders are swept, are crucial, and it seems to me that the OP gave us way too little information for us to make a sensible recommendation.
However I think what posters are doing is fine, stating what proven rigs have got away with using long term .... provided we know other things about their rig, like the above.