I have inherited more than 100.PNG files of products that I need to stitch together for a project. As they are from different photo shoots with various focal lengths they aren't scaled properly. I have been able to colour match everything, so it looks uniform but is there any trick to scale everything to its proper size - by eye or other technique before I start to compose the project files? I am composing the project files in Gimp.
If you want to scale all images to a specific pixel size, using a command line tool like ImageMagick or GraphicsMagick would be the easiest solution. The command would look like:
mogrify -resize 3000x2000 *.png
Make a backup. This command changes the original files. If you want to scale each image depending on the used focal length of that image, things get complicated. Its possible that the PNGs include the used focal length as metadata. Tools like
exiftool can read out such data. You could retrieve the focal length and compute an approximate scaling factor.
But since you want to stitch together all images, you are probably better off using panorama software like Microsoft's Image Composite Editor (very easy to use), or Hugin (more options). Both are free. Hugin is also OpenSource.
The panorama software does the arranging, resizing/bending, blending, and color correction automatically.