< 20x15
The box is placed in the top-left corner of the screen. Looks kind of cheesy.

20x15
The screen scrolls a single pixel (this is the jerking refered to earlier) when you approach the edge.

21x16
The screen scrolls a single tile when you appraoch the edge. All of the playable area is still visible at all times.

Any of the above solutions will work. I prefer the 20x15 level, because one pixel is not that big a deal, and it's not too distracting. Certainly better than 1 tile, I think.

-Nag