Why does my Logo keep jumping in size?

Hi i have had support help me get the menu and logo all sitting where it needs to be. But i am concerned now with the user experience. When i navigate through pages the site ifs very jumpy. mainly the logo will stretch out and then jump back to position. This looks very unprofessional. What must i do to fix this issue? I am using code provided to me from the support team, who have been great!