Related to your question here: Opening an ImageJ "Stack" in QuPath not recognizing channels
Units in pixels could mean anything, so there will never be standard values for them.
The image could have been taken at 2.5X, 20X, 63X, and someone reading this would have almost no way of knowing. The advantage of the units being in microns is that tends to be somewhat more standard (though not completely) and is part of the reason QuPath’s default measurements usually work so well.
Once you find out the correct pixel size, you can apply that to the file through FIJI (Image->Properties), or tell QuPath what pixel size to use as a translation (double click on the Pixel size entries in the Image tab with no objects selected, or use a short script).