Making different sized prints

I'm redoing a logo for a local community/cultural group, as a Illustrator file so it can be scaled. I've never used Illustrator before, but am handy with Photoshop.

I set up the project to fill a standard “letter” page. That's the exemplar, but I will want to incorporate it into other projects or use in different ways.

In particular, I need a sign 24″×21½″. I don't see any option to change the scale of an artboard, and poking around other posts I take it there is no such feature. How should I modify the exemplar into what I want after opening the file?

I don't know how they are printing the file: I presume taking the file to a Kinkos or sign maker shop of some kind, so it needs to be turnkey.

I don't just want to put the logo on another page and scale it as a single lump; I need to adjust the proportions and shift elements to fit the aspect ratio and so on. In this case, the resized page will become the new exemplar, as I'll incorporate feedback about nuances that still need work.