I would like to build a lateral projection of a z-stack with the correct linear size ratio for the final illustration, how can I change the pixels size ratio in ImageJ?
If you want to just reset the pixel size to micrometer:
Get the pixel size/micrometer information from the confocal metadata (when you open your images using ImageJ or FIJI it should automatically populate the information)
When the image is open:
Click Image on the tab > Click Scale (Control+E on FIJI) > A Scale window will pop-up. On the Scale window you can set the X, Y or Z scale accordingly and also modify the Width and Height pixels.
If you are talking about deconvolution and PSF you can also perform those using ImageJ or FIJI.
You can also click on > Analyze tab > Click Set Scale and reset the scale (this is to convert your pixels to microns where you should have a scale bar of known size).