Can you make a poster in Pages 20X30in?

Yes, you can. In the Document Inspector, click on Page Setup.

In Page Setup, choose "Any Printer", and Manage Custom Sizes.

In the Custom Size dialog, click the + sign at the bottom of the list (list may be empty) and name the new entry "Poster".

On the right side of that dialog set your Length and Width. When you are done with that, click OK.

Jerry