There's no logical (or even good.... barely acceptable???) reason why they don't exist. And, thus, I extrapolated... A large wooden shield has 16HP, so one would assume that a large bronze shield should be at around 20-22, given that the small versions have 50% more... Add another 50% for Iron, and you'd be at about 30, and then double that for the Gift - 60HP...
(of course, you could always just have a wooden backing with a thin later of metal)
I don't see a good reason for limiting the availability of sizes either, given that those making the shield would also make the armour and the weapons. And if you've got the iron available to make a full suit of plate, then you could instead make a large shield...