Since: nightly builds only
Returns the overall size of the tab, taking into account all of the contained panes.
The return value is a lua table with the following fields:
rows- the number of rows (height)
cols- the number of columns (width)
pixel_width- the total width, measured in pixels
pixel_height- the total height, measured in pixels
dpi- the resolution of the tab.
dpi may be inaccurate when there
is no GUI client associated with the tab.