Without a lot more information no-one is going to be able to make any recommendations (and if they did you'd obviously not be wanting to rely on free advice of unknown quality)
For instance, are your main chainplates abeam of the mast ("inline" rig) or aft of the mast ("Swept spreaders")? *
Presumably there's just a single
pair of spreaders?
What's your displacement
? Righting Moment? Maximum sail area under plain sail? (or preferably, the luff and foot dimensions of the mainsail
Omce you've gathered this info, and decided what sort of margin of safety
is appropriate for the conditions and rigging life you aspire to, you will either need to visit a decent library and get a book or two to study up on rig calculations, or talk to someone with verifiable qualifications to advise on wire size.
Clamping of the shrouds in the spreader tip is not rocket science and good practice is the same as it was decades ago: if it can be clamped in such a way that the wire is not damaged, and the spreader will not slip if someone heavy stands a foot out from the mast, you're good to go. The other desirable characteristic is that it does not present snag or chafe points to the mainsail when the boom is squared outboard
You do need to make sure the tip of the spreader is raised so that it bisects the angles of the heavily loaded (ie cap) shroud. They should NOT be level, as is seen quite commonly on marinas
*If you have swept spreaders, the spreaders and more particularly the spreader roots must be engineered specifically for that situation, and in particular must NOT have any fore and aft play. The spreader roots need to be much stronger and more carefully designed for a swept spreader rig, because they feed considerable force into the midheight of the rig to push it forward.
However you seem to have forward lowers, which suggests your chainplates are not set up for swept spreaders.