When I'm writing code, I tend to write a very messy chunk and then write a note telling me what that chunk does and how it does it. Is it more beneficial to write less code? Like, could it save space and/or processing power?

Yes it does, since more code = more used memory and the less memory used means more game features and such

You can fix it later (not really)

The primary focus should be to get your code to work at all. After you know it works, then you can optimize it.

Being efficient isn't really important/noticeable unless you're working on a massive complex project or iterating tons of data. In terms of processing. (aka: don't fret it)

I don't think it really matters. Just get it to work, then, after you finish it, go through and make it more simple, like planning out what your limit is.

When writing a program, decide if you want others to see the code. If you want them to see and understand it, then put different functions in different chunks and sub functions under the function to which they belong, but add a space before each sub function line. Also it helps to remove commented lines of code once you don't need it. That will save space and make it look better.

If you are the only one that sees your code, then feel free to make it messy. Remember to optimize!