fixed size for all featured images?

Hi all, Just had a quick question. I have a blog going and i set a featured image for each post. I have different categories based on topic. How do i fix the thumbnails on my featured images so that they are all the same size, right now it looks very unprofessional. heres my website: https://www.djshandsmusic.com
heres an example of what i mean: https://djshandsmusic.com/category/blog/making-your-own-website/

I would appreciate any help here, thank you!!