Generally, if you don't want to pull a permit, you can change a windows height but not its width

This is because of how the wood around it is framed to provide support to the wall above

It depends on how the home was constructed. With stick frames the window frame is built to take the load from the studs above it and direct it around the window and to the studs below. It can be done but it is not an easy job. Going smaller is easy though. Just build a smaller frame inside the existing one.

You can, but you’d need to remove the siding and drywall and reframe the window opening, see a window framing diagram here, can’t cut the king studs or you lose structural integrity: https://www.do-it-yourself-help.com/how-to-frame-a-new-windo...