Hi
@Ploshkin do be careful choosing such a large loom as it can be a mistake, after I let my friend have my little one I bought a 20 inch and found it not good for doing things like scarves etc and of course it also takes up more space and is not too easy to find the right position to use it in.
I was lucky him indoors bought me my 12 inch knitters loom for a Christmas present and I found this much easier to learn on, I sold my 20 inch to a friend and only got a new 20 inch recently which is actually a different make and sits better on a table, now I love both of them, having said that I am already thinking I would like a bigger table loom, I would love a floor loom but I know that is never going to happen.
A 24 might be fine for you but do be aware of the size with relation to your arms and where it will sit.
Are you thinking of buying a stand for it? I had my problem because I could not hold the big loom against my stomach due to an op that went wrong years ago.
As for heddles and shafts they are similar but shafts are found on table and floor looms and are used for patterning, that’s the short answer.
The size heddles you need will depend on the thickness of the yarn you want to use, I use a 10 or 12 more often as I have a lot of 4ply yarns.
Hope all that lot helps, any more I can help with please ask, I am really excited to find some fellow weavers, I have a shawl type cowl to finish off today and will post pictures when washed.