iceblue1188 You can claim a piece of land by using the /f claim command while in a faction, however I don't think it's possible to claim land any bigger or smaller than the default size. You can of course run around and claim all the land around by just typing the command again.
Depends what version. Usually: Older versions: /f claim radius (number) Newer versions: /f claim (faction) (rradius)