HOW TO MAKE ALL IMAGES THE SAME IN BLOG POST

Hi all. Please can anyone tell me how I can make all my blog post images the same size? I have posted two images of the same size to two different blogs but they appear one bigger than the other. There must be a code written in somewhere for one of the images that does not exist for the other image, because when I change the size of one of the images, it remains the same size. Where in the coding section can I add a code that makes all images I post in my blog section ‘FOR THE FEATURED IMAGES ONLY’ the same size???