6x Cube faces at 1592 pixels square = 9552 pixels long and thus 9552x1592 (Remember there are 6 faces to a cube)
The picture is not bigger it is the same but a different projection.
You DO NOT need to convert to cubic to show Nadir and Zenith. Either projection works fine both Equirectangular and Cubic.
The reason to convert to cubic?
: Some viewers require this
:: Because you want to edit a cube face in Photoshop
An old method was to convert the Spherical Equirectangular image to Cubes so that you could edit out the tripod and them reconvert back to Equirectangular after the editing work was completed.
With Panoweaver or Tourweaver you need to tell the software what the projection is by selecting it. In Tourweaver there are many projections to choose from if you have an Equirectangular image you would select "Spherical" and if you are bringing in Cubic then you select "Cubic" from the projection list (last option).
As to why the "Preview" is not working that is another story altogether. It has been asked before and a search of the forum might find the answer.
You could help get an answer by telling us more about the operating system, image and image projection type and size etc.