Let me try to simplify this because there are many good comments but still some confusion...
On the early C-22, "most" boats we typically talk about here, (They may have changed this for the 'new design' @1986 and up, or maybe not until the Mark II), the masthead sheaves are set up for rope to spliced wire halyards. The sheaves are very narrow, only for wire... For Halyards, most people prefer 5/16" or 8mm, but you can still get away with 1/4". For ease on the hands the rule of thumb is bigger is better.
If you want to change to all line halyards you do need to change the sheaves in the masthead and slightly the thin the spacer between the sheave set. All the details can be found in Stingy Sailor's article here;
https://stingysailor.com/2014/05/03/oversize-masthead-sheaves-2/
This article includes the part number for the Ronstan sheaves that will carry an 8mm (5/16") halyard. Keep in mind, this is NOT the "Internal Mast Halyards" modification, your halyards will still be external, including a spinnaker halyard if you choose to use one.
At some point when I finally get to rigging and running rigging on my boat I will be doing Internal Halyards, but not the way you might think... I won't be cutting huge holes in my mast for the exit blocks. I'll be using a different, in my opinion 'safer' method that helps maintain the strength of the spar and actually gives me 5, yes, the ability to have 5 internal halyards. I will probably only use 3 but still have the option for spares.
Another note, I also choose 3/8 for my sheets... again, bigger is easier on the hands. The only drawback is that on smaller winches its hard to get 3 wraps around the drum without the rope kinking and doing all kinds of crazy stuff... maybe thats because my ropes are still new and stiff, not yet broken in... Anyway, I ended up going with a larger self tailing winch anyway.
