Featured image size in posts

Where can i find the exact size featured images display at for each individual post type? so that i can resize an image to that exact size before uploading ?

• This topic was modified 8 years by mradamrothwell.

Also how do i change the default post features image position to make it central ?
i tried

.single_template_default .td-post-featured-image{
margin-left:auto;
margin-right:auto;
}

usually i do that from the modules thumbs setting where it shows all the sizes to turn them on/off.

but some of the sizes shown, such as for the default post show as 696 X 0 , what does this mean, it is 696 pixels wide by any height?

thank you