I use the Kolor AutoPano Giga panorama-stitching software, recently acquired by GoPro, but I have yet to produce a gigapixel panorama like those they pioneered. This brings up an interesting question: given a camera and lens, what would the pixel size of the largest 360° stitched panorama be?

Wikipedia to the rescue: using the formula for the solid angle of a pyramid, the full panorama size of a camera with m megapixels on a sensor of a x b using a focal length of f would be:

m * π / arctan(ab / 2f / sqrt(4f2 + a2 + b2))

For single-strip panoramas of height h (usually a or b), the formula would be:

m * π * h / 2f / arctan(ab / 2f / sqrt(4f2 + a2 + b2))

(this applies only to rectilinear lenses, not fisheyes or other exotics).

Here is a little JavaScript calculator to apply the formula (defaults are for the Sony RX1RII, the highest resolution camera I own):

MP
mm actual 35mm equivalent

MP
MP
MP

The only way I can break through the gigapixel barrier with a prime lens is using my 24MP APS-C Fuji X-T2 with a 90mm lens.

Update (2020-01-21):

Now I could reach 171 gigapixels with my Nikon Z7 and the Nikkor 500mm f/5.6 PF.

Update (2021-01-30):

There was an error in the JavaScript that implements the calculator, it used 4f instead of 4f2, and for telephoto focal lengths, the difference is dramatic. Thanks to users ZS360 and GerladDXB at DPReview for pointing out my error.