The Mandalorian Season 2

[Peelee]

That line wasn't in the script. Mark Hamill thought the cameras had stopped rolling and complained about the oversized helmet. And why is the helmet oversized? Because Luke is a little short for a stormtrooper.

Turns out that mugging ennemy soldiers for their uniforms is no guarantee you'll find your size. Who knew?

A.) Regardless, Luke said it so it exists and I can try to justify it.
2.) It wasn't really oversized; there are generally two types of Stormtrooper armor in ANH, Hero and Stunt*. The Hero suits were made for the heroes - Mark Hamill and Harrison Ford. They were sized for those people in mind. That being said, the helmets are bigger than you would expect when you hold them.
iii.) IRL, no, Hamill called it. You can't see a thing in those helmets. Especially if they fog up, which happens all the time. Fun fact, 501st events will nearly always have handlers.

*Don't ask me the differences. I've read it all and even I can't tell you.