Resize Feature Image

How can I resize the feature image shown on the post?

http://www.tejalskitchen.com/2015/06/sunday-sundae-mango-yogurt/
is the standard post ( wasnt like this before) – I’ve had to manually change in my recent posts on the blog post page under the standard option bit from 400 to 500.

IS there way I can change this in the code for when it posts? to 500 x 750?