Post thumbnail size are not same

First of all i’m new in wordpress and this is my first time creating a website by following different tutorial and i’m not also fluent in english,anyway

I’m having a problem with my post thumbnails,all thumbnails have different size,i tried to search in google and found different plugin like ajax thumbnail rebuild, simple image size,etc but non of them work,by the way im using themify responz theme

i also found this method that i need to put some code in function.php, but when i open my function.php in my editor there’s a message there saying “* Do not edit this file.
* To add custom PHP functions to the theme, create a new ‘custom-functions.php’ file in the theme folder.”

so i also tried that but somehow i can’t make it right

can someone please help me