modifying subcategories images

Hi I want to modify the image size of the subcategories they are too small I want them bigger and in the center and I want all the posts under that subcategory show there. I did create them with the category and then did a parent but I don’t see any option to make that modification ” I don’t know codes”. then I thought about creating the subcategories with pages but then it does not make the function of a category showing all the posts created and linked to it.http://www.izaira.com if you see the categories pictures are bigger and subcategories are very small.