Wordpress功能仅适用于单页（缩略图）

I am trying to control my post thumbnail size on two different pages.

I am using

set_post_thumbnail_size( 875, 175 ); 

To control the size on my index page. And then when they open the post I want the single.php to have the full size image.

Use get_the_post_thumbnail, you would call this in your single.php template and specify the size you want.

You add an image size with add_image_size(); (documentation) and then call on the thumbnail size in your theme with the_post_thumbnail();. You can add as many image sizes as you would like.

So lets create two sizes with

add_image_size( 'home-thumb', 330, 175, true);
add_image_size( 'single-thumb', 875, 300, true );

This will add them and you can call them using using the_post_thumbnail('single-thumb'); since you want the single page to have the full image size you can just use the_post_thumbnail();.

Try this:

if(is_single()) { 
    set_post_thumbnail_size( 875, 175 ); 
} else {
    set_post_thumbnail_size( other dimensions here ); 
}