What size hole do I need for cementing in 2.4m 120mm pine fence posts into clay soil

A fence post has very little vertical force...mainly just the weight of the post and panels it holds. Your driveway slab would work fine as a footing for that. HOWEVER, the primary force on a fence is typically lateral--especially a 6' fence. The reason posts are put into the ground is to counter lateral forces on the fence (typically the wind).

I imagine there are types of brackets that could be used on the concrete to accomodate the lateral forces, but the type you have in your post is not that type...that's typically used to attach structural posts to concrete (it handles the vertical force, but not lateral).

The type of brackets you'd need would be something akin to a shelf bracket where there's a diagonal brace between the two plates.

So, one option is to cut holes in your slab and dig proper holes there. Alternatively, you could find plates designed for this (or have someone weld some up for you).

A third option would be to not build a straight fence, but rather stagger the posts in a zig-zag pattern.

If you already bought (and can't return) 8 foot wooden posts and you want a 7 foot high fence (or "at least 7 feet") I would suggest not burying them at all, as that's doomed to failure. Put metal post bases (or metal posts) in the ground and bolt your wooden posts to them. Otherwise buy 10 or 12 foot wooden posts if you want 7-8 feet above ground and you want them to stay put. Or, bury them 3-4 feet deep and bolt 2x2 or 2x4 lumber to them aboveground to get to 7 feet (or more) above ground level.

If you didn't already buy them I suggest you learn about metal posts, they are usually much less expensive and easier to set, as well as somewhat safer around livestock than PT wood. But you still need enough post in the ground to hold the post in the air stable, and unless you are drilling holes in solid rock to place your posts in, a 1-7 ratio is dubious in the extreme - and so is placing concrete footings above frost line. In reasonably solid ground an 8 foot post can be driven in 2 feet and leave 6 feet above ground, but your corner posts will definitely need some bracing to stay put, and/or will need to be longer.

Wire (mesh or not) is actually more demanding on the posts than a wooden fence - the wire pulls and provides no support - wooden fence sections provide some bracing to the posts.