You are not going crazy; I have suffered under this issue for years on mac.
This is what I do to finally get to export other sizes.
Set the aspect ratio to custom. Type in the size you want for height/width (can be a 16:9 ratio, but still use custom in the aspect ratio). Then press the preview frame size button. Does it display the correct size? Sometimes yes and sometimes no (like 50/50)
If it does not display the correct size, you will notice upon escaping the preview window, that all your settings have reverted to the size displayed as opposed to the size wanted.
Continue doing the exact same thing until you get a preview size that is correct. This can sometimes take 3 or 4 tries. I have never had it fail to finally give me the size I am looking for.
Once you get the correct preview, then you can chose OK and start the export.
I always use image sequence exports. This allows me to navigate in my finder to where the images are being stored and I can check one of the export images to make sure the correct size is exporting. If the preview page was correct, you'll find that the images are correct too. If you export full animations without image sequences, then you have to wait until it is completely done before you can check, which can be a huge time waster if it is exporting incorrectly.
Good luck and yes this a long time bug...