May I remind you that the question includes the following:

"if I know no other info about this image"????

That is why there is no answer however you want to slice it. I made the same error because I had not paid attention to the original question and like many went into you can using this or that... There is no 'this' or 'that'

A pixel has no size per say, it just the smallest amount of data used to create an array that in turns becomes an image. Sensors have a pixel per inch density that is then transformed into another pixel per inch on display, print, projection - whatever. None of these pixel has a size EVEN if the original gizmo to create that pixel had a microscopic size.

It seems that manufacturers are now leaving anti aliasing filters off many new cameras. Apparently, with new sensors, moire is less likely to be a problem in future. Until recently it has been easier (much easier) to sharpen (in PP or in camera) that tiny little bit extra to overcome the difference between with anti-alias or without, whereas moire, which is the reason for including an anti-alias filter, is very difficult, if at all, to get rid of. However, as the pixel race resumes, and sensors become more crowded, I believe moire will continue (or return) as a problem.
Have Hogs any thoughts on this, based on practical experience, or just logical thought, perhaps on the problems of moire and the advantages ar otherwise of anti-alias (low pass) filters?

The Nikon D810 had a 36 megapixels Sony sensor. The same sensor is in the Pentax K-1. I was surprised that the D850 of 45.7 megapixels, is not a Sony sensor. I don't know if the upcoming D-6 sensor will be Sony? Sony probably will not sell their newest 61 megapixels sensor that's in the a7riv. They had to make a full frame mirrorless camera that had more megapixels than the Canon and Nikon. It's called the megapixels war. It sells cameras. I have heard that Canon has a 100 megapixels sensor. Not yet in Canon cameras. Sony does not have one that high. Yet.