For any application using photographs for measurement (Photogrammetry) the size and resolution of the image has an impact on the accuracy of the results.

When using 360° Panoramas for measurement the size or the Equirectangular projection, and projections derived from it such as Cube Faces, Sphere, Little Planet, excetera, will impact on the accuracy of the results.

PTGui uses the following formula to dervis the size of the Equirectangular at 100%:
w  =  2π  x  f  x  (px/mm)
where:
w  =  width of the equirectangular in pixels
f  =  focal length of lens in mm
(px/mm)  = pixel density of the sensor where:
px  =  pixel width of the sensor and
mm  =  width of the sensor in mm

The size for the Equirectangular can also be read from tables derived from this formula.

The formula and tables should be used as a guide rather than definitive as the value for f (focal length of the lens) quoted for a particular lens is not always accurate.
The focal length of the lens used by PTGui in the formula is determined when Aligning the images and generating Control Points as alluded to in entry 3.2 in the PTGui Support FAQs and can differ from the value quoted for some lenses.

For example:
The Pergear 7.5mm Fisheye (Nikon Z fit) has a quoted focal length of 7.5mm, but the focal length determined by PTGui is 10.5mm.
Using 7.5mm in the formula gives an Equirectangular of 7900 x 3950 = 31Mp, whilst using 10.56mm in the formula gives an Equirectangular of 10900 x 5450 = 59Mp;
The TTArtizan 11mm Fisheye (Nikon Z fit) has a quoted focal length of 11mm, but the focal length determined by PTGui is 15.0mm.
Using 11.0mm in the formula gives an Equirectangular of 11580 x 5790 = 67Mp, whilst using 15.0mm in the formula gives an Equirectangular of 15760 x 7880 = 124Mp.

Conclusion: If you are looking to purchase a new camera and/or lens the formula and tables should be used as a guide, but it is preferable to obtain the data from an actual user if possible.

It is also important to give cognisance to the pixel density (px/mm) of the sensor as a larger sensor does not necessarily mean a larger Equirectangular and it will often be the case that the reverse is true.
For example:
A 24Mp DX sensor (23.5mm x 15.6mm) will have a pixel density of 255 resulting in an Equirectangular of 16820 x 8410 = 142Mp with a 10.5mm lens;
A 24Mp FX sensor (35.9mm x 23.9mm) will have a pixel density of 168 resulting in an Equirectangular of 11080 x 5540 = 61Mp with a 10.5mm lens.

However, a larger sensor will enable a longer focal length lens to be used for a given shooting pattern.
For example:
A 24Mp DX sensor (23.5mm x 15.6mm) will have a pixel density of 255 resulting in an Equirectangular of 17646 x 8823 = 156Mp with an 11mm lens;
A 24Mp FX sensor (35.9mm x 23.9mm) will have a pixel density of 168 resulting in an Equirectangular of 17900 x 8850 = 160Mp with a 17mm lens.

From communication with PTGui Support I have learned that the optimum size for the Equirectangular is not “set in stone” and is a possible interpretation. as the lens and the Equirectangular projection do not have a uniform resolution.
The Equirectangular can output any desired size by changing the value in the “fit at:” box in the Create Panorama dialogue.
Increasing the value in the “fit at:” box in the Create Panorama dialogue does not appear to have any impact on the visual appearance of the Panorama, but does reduce the angle subtended by one pixel so may be a way of increasing the accuracy of measurements made using 360° Panoramas.