First of All Please use image 870px width & 500px Width.

We have 2 solution for Featured image on the display post page.

1- You don't want to apply any size just show which you have upload , Then add this code in Your custom.css file, from WordPress Editor.

.thumb img 
{
width: auto !important;
}
.thumb {
    background: none repeat scroll 0 0 #fff !important;
}
(Note :Replace #fff with your own backgroung color.

This one is best. )

And 2nd Point is this You want to show image fit on thumbnail image area, So then apply this code in you custom.css file. 

.thumb img 
{
width: 870px;
height: 500px;
}<br>