That's in the Manage Maps item under the Maps Menu. Right now there is only the ability to select the upper and lower lat/long and the zoom steps to get but it will download every map that makes up that area at every magnification step that you check.

I would like to see a pixel-doubled version of the maximum zoom (maybe a menu choice for "Zoom 0.5") since the resolution of the screen is so high that I almost have to squint at it even on the closest zoom level.