Inspect Elements (FireFox) Changing Code

I am trying to change the height of an "input field" on a "Slider Login Plugin" I have installed on my site.
I can find and change it with "inspect elements" in Firefox but I don't know how to determine which file need to be changed. I can see its "line 134" but not sure how to determine what file it is or where that file is located.
Any help would be appreciated.

Thanks JIM