i'd go for the biggest. personally, i have a 27 inch internal width panel, and it still fits through all the doors, except the closet, but it's never going in there. the reason for the biggest is that hopefully some day you'll get a bigger monitor, and when you do, you'll have the space you need. the only downside to this at the moment is the fact that your monitor will look incredibly small till you upgrade.
so i guess it depends on if you plan to upgrade within the next year or if your up to starting from scratch if you do upgrade.