featured image size and categories template

using highend theme. on my blog posts page, i am using blog-classic, small. but my featured images are all different sizes(mainly the images that are tall). i would like for them all to be the same size even if they get cropped just for viewing purposes until they go into the post. please let me know. also, when going to a category, the template is different from what i want, i think its on minimal, how do i change that?