I have been using Hugin as a panorama stitching tool. Its help file states that the equirectangular mode that it supports is a spherical mode. The last tab is the stitcher page and it has dimensions that you can enter for cropping. It also shows how many pixels the panorama has before cropping. I am thinking that with this info you should be able to output a 2:1 ratio equirectangular mode panorama. I hope that this helps you.

You would first need to shoot enough frames to cover 360 degrees. Remember that you may have to shoot in portrait mode and have two or more rows of exposures to get enough height to allow a 2:1 ratio when going 360 degrees. I would normally sweep 8 exposures overlapped about 30-40 percent (10mm crop sensor lens) to get 360 degrees. Don't forget to measure the nodal point of the lens and buy ($1200) or make ($10) a panoramic head so the camera always rotates about the lens nodal point to avoid parallax which will make it difficult to stitch the frames together if there are objects close to the lens.

Depending on your camera you could end up with a 15,000 x 7500 pixel photo (112 megapixels). I would then downsize it in something like Adobe Lightroom to the exact pixels you need.

Sincerely, Steve Pituch http://hugin.sourceforge.net/