What determines the form?
I'm never actually sure if the whole "protector" thing is an extrapolation, because Luna's Patronus is a rabbit, and Ron's is a terrier. I always kind of thought that it has to represent yourself in some way too. I think that it has to represent you as well. I always thought that Hermione's Patronus was an otter because they are very clever animals, as is she. (And it was JKR's favourite animal.) Ron's is a loyal terrier. So, like they represent some of your qualilities, I suppose. But then again, I suppose people would find comfort in security in their good qualities. Cho's Patronus is a swan, which is often a symbol for beauty. I mean, I know that they can be super visious and all, but I think that again it is representing Cho. It just appears that many people's Patronus represent themselves in some way.
Is a Patronus always 'life-sized' and if so, is a larger animal Patronus harder to conjure?
My guess for this question is yes they are life-sized and no its not harder. But i'm not sure.
Nothing is mentioned in canon of corporeal Patronuses swelling or shrinking in size. The books simply describe whatever animal it is, so I presume they are always life-sized regardless of the situation.
Every animal's size is different. A cat is quite small, but a lion is bigger and has that much more light. Would the size affect the strength of the patronus?
I don't think size has anything to do with it. What matters is the skill and strength of will of the wizard/witch casting the spell. So a very skilled wizard who's Patronus is a mouse would have a stronger Patronus than a novice who casts a humpback whale.
can you will your patronus to change sizes? To get smaller if you just needed to send a message?
I've always imagined Patronuses to be the same size. The only thing you can do is change their strength and/or send a message.
When are they taught normally at Hogwarts do you think? A group of fifth and sixth years didn't know the charm (then again, they've had some awful DADA professors). Is it covered seventh year? When would make sense?
I get the feeling that the Patronus Charm isn't normally taught at Hogwarts at all. The majority of Death Eaters don't know how to cast a Patronus, but this isn't out of any ideological opposition to the spell. They were Hogwarts students once too, so they must have just never been taught.
The only people in canon we see casting Patronuses are members of the Order of the Phoenix and Dumbledore's Army. This suggests that they learned the spell through their own initiative, not from Hogwarts.
Besides, I can see why the Ministry of Magic would be reluctant to endorse teaching the Patronus to students. Why teach students how to defeat Azkaban Prison guards, who are of course, protecting the community? Also, teaching the Patronus could imply that Dementors are roaming about and need protecting against, which would be alarming.
But I do think that the Patronus should and probably would be taught post-DH, since Kingsley is in charge and all of the other happy ending stuff.
Tim the Enchanter
Perhapes in the case of Luna, her greatest form of security could have come from just being herself. Or it could have somehow been a symbol of her mother, who was apparently quite...unique as well.