why is "Change output size to match source" greyed out

I was hoping there would be a better way. I remember doing something similar several years ago in an older copy of premiere pro, and was able to use the option to "Change output size to match source" in the output tab. I have no idea why they left it in the option menu since it's never avaliable to use. That just doesn't make sense to have an option that nobody is allowed to use anymore. Adobe needs to fix this and make it so that you can just crop and export at that cropped size without the black borders. That would be easy to do if they wanted to.

Thanks for trying to help, I appreciate it.

Thanks for clearing that up about pixel dimensions, I didn't quite understand what you meant by that. I don't use them for anything special, but from time to time I'd like to be able to save a small clip of a portion of a video to pop up over a video to make a point. Or to cut out the sides of a video that was shot in portrait mode instead of landscape mode then exported with a background in say 1920 X 1080 to get rid of the background, or black bar sides. I know you can mask out what you don't want in premiere when you do this, but if you use the same clips in more than one video you have to do it over and over and over however many times you use the clips. If you just are able to export the clips in the size and format you want, without the black bars or background it would save lots of steps and repetitive work down the road. 

This is what I'm talking about. It's too many steps to take. What you're describing is sort of a "one size fit's all" approach and that isn't what I need because it just won't work. Something that fits "most" won't work for everything. (for me, I find that things that "fits most" never fits what I want) I'm not asking about pixel dimensions, I don't think pixel dimensions would affect what I'm wanting to do, square pixels would work perfectly fine as the shape is rectangle, not an odd shape with curves or anything. To accomplish what I want, these are the steps I have to take. And it's really quite comical when you think about it that so many steps have to be taken for such a trivial task as this.

It should be so that you can just set the crop, then export it to that size without black bars, without all these steps.

Hello Ann,

This is an example I made, not the video I was asking about. I fixed that by subtracting the cropped dimensions from the original dimensions then resetting the video size then exporting it. A lot of unnecessary steps to do if you have to do this on a regular basis. I already deleted the original source files for that video. So I captured a bit of video to make this example from a comercial on tv. You can see that the option to "Change output to match source" is greyed out. 

What I need to know is how to just drag the crop to the portion I want in the source tab then export the video to that that size without any black bars on any side. Without having to subtract the difference, and "do the math" and then change anything in the "Basic Video Settings" before exporting the video. 
One should be able to do it just by draging the crop to size in the 'Source' tab, then choose "Change output size to match source" in the 'Output' tab and export it without have to figure out a specific size and then change values elsewhere. That is slow and tedious.