There are 68 module positions. If no module is published to a position, it will not appear, collapsing the entire area.
There are non-standard elements that are injected into the template grid structure when enabled via the template parameter, such as the logo into header-a. These features stack vertically with any modules published to that position.
Control all modules positions with Gantry, in terms of widths within the module row, and left/right/middle placement for the mainbody/sidebar area.
Gantry Framework : Layouts Documentation
The below diagram is of the Module Variations page.
This is the Popup Module feature. Assign any module to the popup module position, and ensure that the Popup Feature is enabled in the Gantry Administrator.
You can configure its height and width from the Gantry Administrator.