So it's either a size 2 or 10!! I guess you can go smaller on a hair. It's just taste and fancy I think. Too much choice these days!
I wouldn't worry too much about size of hook, more presentation, Barbel are pretty unfussy eaters and will take anything on offer almost, I find when they are competitive feeding it matters even less, but nowadays sadly we don't enjoy shoals of Barbel like we used to, unless we are on the Wye or similar, so my approach is different depending on where I am fishing.
Feeder fishing on the Wye or Middle Severn is different to the Lower Severn or Warks Avon, where I am fishing mainly for solitary fish. Feeder fishing I would hair rig a say 12mm pellet or boilie on a size 8 or 10, when the bites dry up, which they do, I will fish smaller baits with smaller hooks, say a 8mm pellet on a 12 , 'fishing for bites'.
On the Lower Rivers where the numbers are less, at least more spaced out anyway, the potential for bigger solitary fish is very real, so I will use a bigger bait maybe 2'' cubes of meat, this will ensure I create a sort of 'feature' in itself that any passing big girl might be tempted. A large bait satisfies the effort vs reward, especially in cooler months, but will also work in summer when appetites are greater, and you can discourage unwanted bites from other species, but the problem here is Eels are a nightmare when using meat, and at least a size 2 hook is just too big for them.
I know a very good Lower Severn angler who use nothing but 14mm hallibut pellet summer and winter, and catches more barbel than most , why?
Because he believes in what he is doing, that's why.