i need help with featured post Images Size

Hello Guys ,
Since i'm new in wordpress, i have already instelled a template for my new wordpress website, but i got some problems with Featured image posts size, every post has another size image, i would like to set a dauflt size for all my post images size, how can i do this please, i tried to fix it from ----settings--- media---thumbnail size, but nothing change, i think i have to change something in template files or ?


I can't figure it out. Thank you so much!