Structs and global arrays

Hi. I've checking with JASS Helper Manual But I don't understand it.

I want to have all the data within a struct is an array. I can do this with a predefined array size, but I wanted to know if I could change the array size from another function (not constant).

But, I want to be able to use the struct as an array with the size declarable from within another function. I could just make the array the max size but I thought I’d save some memory if I was able to use only what I needed.

Also, just to make sure, a struct outside a scope are global and can be used by other triggers. Or should I make it into a library?

Edit: I've ended up going with a predefined array size.