It is possible to code will be able to increase functions.php to alter the default thumbnail size? I understand will be able to alter the size in configurations however i require a function that is going to do that instantly for my theme. I am not mentioning to publish pictures.

You cannot change them but list of positive actions is register your personal image dimensions rather and employ individuals inside your theme. See [cde] for documentation. Then rather than while using built-in thumbnail dimensions, only use that custom one you define inside your theme.