Need to change image sizes in Magento template
I'm wondering if someone can tell me what file(s) I need to edit to enlarge my photos, but not lose clarity in my new Magento template. I find the pictures too small and the template automatically places borders around them, so half the pictures have white space around them. This occurs on the home page, and on the product list pages. I can't go and change thousands of pictures manually, so how do I do it without them looking fuzzy? Should I get rid of the border as well and if so, where is that done?
I am very new to this, so any help or suggestions would be GREATLY appreciated. I'm pretty sure I need to edit css files, but I'm not sure which ones.