When you export, do NOT check the box next to "Resize to Fit" in the Image Sizing section of the Export dialog box.
By the way, File Size is completely the wrong measure to be looking at, there is no way to export and have the file size NOT change; you want to look at the number of pixels (width and height) in the exported photo.
Yes but that section is not activated for some reason. I can't click on it
Please explain further ... or show me a screen capture of what is happening.